I would get a new graphics card if I were you, either a 9800pro or 5900.
Then I would buy a new processor as cheerios pointed out.
You should get the fastest AMD with a 266MHz fsb I think thats 2400. Otherwise you'll have to buy new ram, which seems a shame as you have 768MB already.
Pentium 4 2.8 with M0 stepping.
Only air cooling tho