I've only seen 1 system rig up so here's another one for you.
NewEgg.com under $900
With a little overclocking knowledge and research... this should net you very close to 3.5Ghz.
This is based on no recycling of old parts. However, if you are going to use some old parts... you could possibly invest in a better graphics card X1950XT/XTX or 8800GTS/GTX
Good Luck!
heh, you'll need an optical drive!