actually, cpu can make an impressive difference. The other day i got my hands on a 1.6ghz P4, and swapped it with a 2.1ghz celeron, and it made the pc 2x as fast, the loading bar went acrost 4 instead of 8, halo took way less time to run, it quit lagging, it was quite impressive.
and i hope you dont plan to buy the amd and implant it into an intel system (a freind of mine did this, got a skta cpu and wanted to put it in a skt 370 mobo. long story short, he ruined the cpu and trashed the socket.
OOH BTW. i have that cpu, and it is great, getting 800MHZ+ overclocks out of it.