To clarify a little. You should have an internal 3.5" drive bay avalable. You should have an external 3.5" drive bay avalable. You have a 5 1/4" drive bay avalable so you do seem to have 3 bays avalable. The two 3.5" will take a hard drive. The 5 1/4 will take a hard drive with an adapter (that cost just a few dollars). You might as well just put the hard drive in the internal bay next to the other hard drive. You could fit three more hard drives (total of 4) in your computer if you wanted, but that would fill all your drive bays.
You have two diffrent controlers. The first is a pata controler and it can take two drives (on the same channel/cable). Most computers have two but yours only has 1 (1 channel can take two drives). You dvd drive is using this channel so you could add one more drive to it. It would have to be a pata drive (all the ones you are looking at are sata and would not work).
your second controler is a sata 1 controler with 4 channels. Sata can only take one drive per channel. One is being used by your current hard drive. You could add 3 more sata hard drives to the other three channels. That means your controler can handle them.
So you have a total of 6 drives that can be controled, 2 pata, 4 sata, but one pata is used by your dvd drive, and one sata is used by your current hard drive, leaving4 controler spots open (1 pata and 3 sata). That is how many drives your motherboard can control.
On top of being able to control it, you need a place to mount it. You have 6 drive bays, 2 5 1/4", 2 3.5" external, 2 3.5" internal. One 5 1/4" bay is taken by your dvd drive. One 3.5" external is taken by your card reader. 1 3.5" internal is taken by your current hard drive.
So you could put 6 drives in your computer (if you removed the card reader), two optical and 4 hard drives or 1 optical and 5 hard drives, and could control them all. I'm not sugesting taking out the card reader, I'm just saying that is the max number of drives your system will support.
Be aware that pata and sata drives are not the same and are not interchangable. You can control 2 pata drives, and 4 sata drives. Most opticals are pata. Pata and sata hard drives are both very common. You might as well leave the second pata controler spot open for an optical (there are only a few sata opticals even avalable). Sata is beter for hard drives anyway (keeping that spot open will not stop you from adding drives now as you have sata open). Sata is the newer, beter, more capable technology for hard drives so you might as well get them. There is not really much of a speed diffrence between pata and sata hard drives, as the mechanical limits of the drives are slower than the max allowed by any of these standards. You might get a little performance increase from sata from the way it handles data and the way the controlers are incorperated. Not a raw speed increase, but rather slight performance increases on how the data is handled through the controlers and motherboard.
So basically no problem adding a hard drive now and a couple more in the future if you like with the exception of the power supply.
With 1 hard drive, and no video card, your system takes about 200 watts (ball park figure based on what I got from the power calculator I linked to with 3 usb devices).
Your video card can take up to 75 watts without an extra power conector (thats what the pci 16 standard allows for, a maximum of 75 watts).
You are already near the 300 watt max of that power supply (and I guarentee you that pos cannot actually put out 300 watts continouslly, cheap power supplys often have exagerated ratings). Even an old low power radeon 7000 takes 30 watts, and old 9200 or 5200 takes 40 watts. Most newer cards are going to be closer to that 75 watt limit (higher if they have an extra power conector). Each hard drive maybe 15-20 watts. So you got a videocard and a another hard drive and are pretty close to 300 watts, whcih your power supply cannot handle already, and you plug in a firewire device (that can take 4 times as much power as a usb device) and bang goes your power supply!!!!! Just a warning...... Am I trying to scare you? Yes....
In reality, those are max powers. You computer probably doesn't use half that most of the time like when you are web surfing or typing a document. Its a good thing too since that power supply probably can put out 200 watts at best if even that. You could add that hard drive and the computer would run just fine, till you try gaming, or transcoding a videofile and websurfing at the same time or other kinds of multi tasking.
It will run fine till you push your computer and or plug in a high power firewire device and little upgrades like a stick of memory or a fan makes it even worse. There is no perfect solution because we never really know exactlly how much power your computer is going to use (you can have two idenitcal cpu's and one will use a little more power than the other). even with the power supply calculators, they are only approximates.
My opinion, you are seriouslly pushing your luck with that power supply. It could run fine, especially if you never push your computer, but I think you are taking a big risk. You can check it out. Bare minimum, run software like sandra or whatever that can report voltages and see if your voltages are in spec. I would do it right now infact and see how it is doing with the videocard installed. It's probably going to be ok, but try running 3dmark or the sandra burn in wizard or something like that and see if the voltages stay stable.
As far as the drives, many of the main stream 7200rpm drives have similar performance. I usally go for the best price per gig. You need to go to raptors or something like that to get a noticable performance increase.
As far as brand, out of the ones you are looking at, I would avoid maxtor. they have a reputation for poor reliability (drives dying) and aparently they don't have too much confidence in their drives either as most have a crappy 1 year warranty. Seagates are much beter quality and have a 5 year warranty. I really don't have an opinion, good or bad about western digital, perhaps someone else can coment. Also, you are looking at both sata 1 drives (rated at 1.5 MBps) and sata 2 drives rated at 3MBps (out of the ones you linked too). Your controler is sata 1. While I think most sata 2 drives are backwards compatible, I have heard of problems once or twice. Perhaps someone more farmiliar with the two can coment.
Hope that wasn't too confusing. Also, if you are in the US, consider checking local ads (bestbuy, circuit city, the office supply stores, fry's and micro center if near you). Those prices are kind of high. Hard drives are one of the few items that you can actually get a lot cheaper locally when on sale.