There are defanatlly reasons to go bigger. First, power supply calculators are just a guideline. Some of the equipment might use a little more or a little less than the power supply calculator states. Hard drives and optical drives would be a good example. The values are averages but some drives may take more. Second, it is not good to run your power supply under full load (like using a 350 watt power supply for a system that uses 350 watts). Even some of the good ones wont do it and or voltages can become unstable. Another reason is that if you go with a power supply that is just big enough, it leaves you no room for upgrading/adding stuff. If you ever decide to overclock, that takes more power too.
There are lots of reasons to go overkill on the size of your power supply which is why people usally recomend it. I personally agree. If the calculator says that you use about 342 watts, you could probably get away with a 400 watt minimum but I would recomend at least a 450 watt, not only to be safe, but to give room for a little upgrading, more usb stuff etc.
Enermax(coolergiant), fortron (sparkle), antec, ocz, pc power and cooling are all good brands and there are probably others. With antec, smart power are not as good as true power. With ocz, mod stream are not as good as power stream. With enermax, whisper series are not as good as the other ones. When I say they are not as good, I do not mean that they are poor quality. I mean the the higher ones will probably have a little higher true power output compared to the lower ones (If I didn’t think they were good I wouldn’t be running an enermax wisper in one of my computers and an antec smart power in the other). If the price is right, the lower models work just fine.
Here is the enermax I have
here are a few more good ones for under 100$
There are of course lots more good ones both under and over 100$
Here is some very good information on power supplies as well as more recomendations
Fyi if you are going to get a 600$ video card then I’m assuming you are not broke. It might not hurt to spend a few more dollars and get something in the 500 watt range. there are good ones that you can get for about the same price as the ultra (see both my recomendations above and the link I gave you). The ones I recomended are not nesasarily the ones I consider best. I just picked some good ones off new egg.
One last thing. Be certain what conectors you are going to need and make sure the power supply you get will suit your needs. Some of the neww stuff uses diffrent conectors (like 24 pin motherboard conectors) and not all power supplies will work anymore.
Fyi ide and sata are diffrent kinds of hard drives. If your system was originally a factory built one, its probably got regular ide drives (pata). If it was custom build, it might have either.