From first hand knowledge of doing this 10 months ago I can tell you it's a night and day difference!!! I couldn't believe how much faster the new system was...My first reaction was Whoa!!!
Example: Creating a new backup img for my OS partition.
Old system with the P4 2.66Ghz / 590 FSB,
35 mins 
New system with Core 2 Duo E6700 @ 3.3Ghz /1300+ FSB,
3 mins
Gaming...it's just Crazy good now!!!
Sandra Benches put it at 86% faster than my old system...