IMO, the difference in performance is quite neglidgeable. All I want is a CPU that can
download a few files in the background, browse multiple web pages, check e-mail, use 1, maybe 2 IM's, play a video in Winamp, and maybe have some other programs open in the background.
I think that an Athlon 64 or a Pentium 4 could handle that. But the Athlon consumes about 100W less and is generally cheaper than the competing model, so Athlon 64 is the best for me.