Oh, boy! This is going to be a challenge. Your lucky I love a challenge. I think it is doable but you will need two things: (1) Be able to continue cough up cash for the upgrades, and (2) have lots and lots of luck.
Upgrade 1: ASRock 939Dual-SATA2
@ $70. This board will give you both AGP and PCIx, which is essential to make this madness work. AMD Athlon 64 3800+
@ $290. This is a very nice processor for your future gaming machine.
NOTE: You better hope and pray that your old RAM will run it or your without it until you can afford the next upgrade.
NOTE2: With the old RAM and video card bottlenecking you won't see any performance increase.
Upgrade 2: CORSAIR XMS 2GB (2 x 1GB) DDR 400 (PC 3200)
@ $240. Model: TWINX2048-3200C2PRO. Time to get some RAM that will let that processor of yours run. You won't be getting the increased bandwidth of DDR2, but with PC 3200 running in dual channel mode you should have enough. 2GB because MMOs use loads of RAM. The difference between 1GB and 2GB in WoW is huge.
Upgrade 3: XFX GeForce 7900GT 256MB
@ $300. This thing will give you the same performance as a 7800GTX but while using less power. It will give you better odds that the machine will run on a power supply you got running around.
NOTE. By now you will be relying heavily on the 12V on your power supply. Older power supplys usually don't give enough amps on the 12V rail to handle modern high end computers, so you better keep you fingers, toes and anything else you got lying around crossed, hoping that the machine will still run.
Upgrade 4: ENERMAX All in One Noisetaker Series EG701AX-VE
@ $150. Time to get a power supply that surely will run this beast of yours. Antec Performance I P180
@ $130. A case that will keep things both cool and quiet. Besides, it got filters in front of the air intake, which will be a great help in keeping the computer dust free and alive. They are also some of the easiest filters to clean I've come across. Might want to add another 120mm fan at the front intake, though.
A 20GB 5400rpm hard drive just won't do for modern games taking over 5GB space. Western Digital Raptor WD740GD 74GB 10,000 RPM
@ $160. This thing should do a world of difference on loading times. Put Windows and the games on this one, and everything else on: Seagate Barracuda 7200.8 250GB
@ $100. Here will come all other programs, music, and anything else you'll end up filling your harddrive with. Plus, it comes with a 5 year warranty.
Upgrade 6: SONY Black IDE DVD-ROM Drive Model DDU1615/B2s
@ $25. To be able to deal with the games that are on DVD. Creative Sound Blaster X-Fi XtremeMusic
@ $ 120. you'll also need a good soundcard to avoid stuttering in modern games while using stuff like EAX4. And the sound quality is way beyond and onboard solution or old Soundblaster Live.
NOTE: You'll need Windows Xp SP2 to properly run some of this hardware.
I don't know how much of your old rig is left anymore but to do this monster justice you might want to continue you upgrading with a new monitor and some good speakers. Maybe a sleek mouse and keyboard.
Wow, talk about extreme makeover. I should get my own TV show.
This will be a very good gaming rig, that should be able to handle anything you'll throw at it.