From the whatsnew.txt file:
1) Athlon users should see a 20% to 30% speed increase.
2) Pentium, Pentium II, Pentium III users should see dramatically WORSE
performance. Run version 23 instead.
The current version is 24.11 and it also has support for amd64 chips in both 32 bit and 64 bit windows. you can get it from http://mersenne.org/gimps/.