Oh god... where to even start. Everyone will have different preference.
I say build a whole new on. Get a credit card and buy EVERYTHING online usign
www.pricewatch.com and checking before you buy with
www.resellerratings.com
Do you know how to overclock? If not, would you be willing to take the 10 minuts to learn how?
I recommend you get a Athlon 2500+ Extremely cheap and can be overlcoked SO easily to 3200+,
Get a Nforce2 board. The FIC AU13 is cheap/awsome.
Video cards are WAY out of whack right now.... I don't even know what to buy.... For ultimate power get a Radeon 9800. Radeon 9600 for best price/performce. Or go with a good old GeForce 4 is you like. Dont get a Geforce FX, or anything that has the name MX or SE in it's name. (like Geforce MX 440 or Radeon 9800 SE)
um..... Get a good PSU. At least 350 watts. Make sure its a good brand like Enermax, ThermalTake, Antec.
RAM. To compliment your 2500+ I would get PC3200 RAM. Definatley get at least 512MB, and get 1024 if you can afford it. ONLY get good brand names like Geil or Corsair. Others are out there, these are my favorites.
GET A GOOD MONITOR if you don't already have one. Don't waste $$ on a LCD. I would get a flat CRT 17" or bigger. Read up on what dot pitches are good and what resoltuions it can display. Obviosuly if you get a radeon 9800 or soemthing, and your monitor sucks, your whole computer now sucks too.
Can't think of anything else right now.....
I know people may flame me for my opinions, how dumb taht may seem, (and my spelling, gosh!) anyway, I have built 3 computers in the past 3 months, and 1 more next weeks. This has been the setup for all of the rigs, and it hasn't failed me yet. 1 friend of mine bought a crappy monitor for his great custom-built PC and that is why I brought that up...lol