Depends on what your old AMD system was. general windows-usage wise, you'd probably not notice much difference between an Athlon XP2000+ with DDR266 RAM, and a P4 2.4C w/ DDR400....
Diff. would only really be noticeable in fairly recent games, and video rendering etc - depending on the video cards of course.
If you were comparing benchmarks, you'd probably notice a fair increase, but it wouldn't 'feel' much faster in windows if you were just typing letters and playing tetris.
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$hit Happens. I just wish it would happen to someone else for a change.