Yeah, I agree with nitestick. However I would like to add on. AMD is releasing their new socket real soon (a week?), which could lower prices tramendously. Also, in a month (or 2?) Intel is coming out with its new processors, dubbed Core 2. Core 2 is supposedly going to top the highest Pentium D by 40%. In reality it's probably closer to 30%, but still amazing. Either way those are somethings to think about.
If you cannot wait, get the NF4 DFI Lanparty Ultra-D and 7900GT like nitestick said. But I would get a Athlon X2 or dual core Opteron. If you are into overclocking, the Opteron will do you much good. If you are not interested in overclocking get the Athlon X2.
Athlon X2 4200+