There's no perceivable difference. I had an Athlon XP 1900+ with 2x512 MB of PC2100 at 2.5-3-3-7 in one of my computers. My girlfriend only had 512 MBs in her computer, and The Sims 2 ate up every last byte of that, so I swapped her 2x256MB of PC3000 2-2-2-5 RAM with my slower set of 2x512 MB. Now my 1900+ feels the same in Windows, but MUCH slower when loading and exiting games, while her computer is much snappier than before.
According to benchmarks, 2.5-3-3-7 is about 7% slower than 2-2-2-5 in 3DMark 2001 SE. I can't see the difference.