Are there any parts you wish to salvage from your old system and or will you keep the old system intact as a second computer?
I like to line up a basic good computer, see how it falls budget wise, then upgrade where I feel it nesasary.
Here is a nice basic system.
athlon 64 3000 114$
abit motherboard (didn't bother to pick the best model for the money, just picked one for now) 90$
1 gig corsair valueram 80$
antec case (I prefer full tower that is well built, there are endless options that are to an extent, prefrence here) 100$ (got two for 50$ ea on sale after rebate, on with and one without window, this is just for refrence). Actually the one I found for refrence is 140$ with usable power supply (actually a good power supply but maybe not sutable for dual core with a high end vid card or sli).
Power supply (if you were to get the above case you wouldn't need one). 80$ I like this one because it has a high single rail 12v which is good for overclocking, dual core cpu's, sli, high end vid cards (not all together though).
Actually, this power supply as well as the on included in the above case are overkill for a basic system but overkill in a power supply is a good thing and it give room for upgrading. Depending on what equipment you settle on and or what you think you might upgrade to in the next few years, it might be worth getting bigger.
Optical drives, I'm not even going to touch that question at this point as that is the specialty of this site and it is a topic of its own. You can get any of several good dvd burners for 40-60$ on newegg and a good dvd rom for 25$ plus maybe 10$ max shipping so figure 80-90$ for two opticals.
hard drive. 130$ for a seagate 300 gig (hard drives are one of the few items that you can get locally cheaper on sale, this is just for refrence and ball park priceing).
video card, ranges from under 100$ to a fortune. I'll leave that one to the experts and just throw a 100$ budget in there).
Thats about 700$ for a nice average system. that assumes that you have monitor, printer, keyboard etc from your old system. Be aware that your current copy of windows xp (or whatever you run), if oem very well might not work on a new system and a new copy of xp is about 80-100$.
So how does that price seem and how much more or less would you like to spend?
Just my personal opinion based on taste, and personal prefrences, on upgrades.
abit fatality mother board, add 100$
over clockers ram or 2 gig of ram add 60-100$
sound card (you can get a nice SB audigy 2 value card for 38$ at office depot (clearance) if you are lucky and there are any in your area, otherwise, there are some low end cards for 30-40$ that will beat your onboard sound and prices go up from there.
tv tuner/capture card, minimum of 40-50$ for a cheap one and up.
raid (your motherboard should handle it fine, you need a minimum of 2 identical drives so cost of drives).
There are lots of options and areas to upgrade plus I probably missed something. Based on this though, where would you like to upgrade, how much do you want to spend, and what do you see that you like?