I have read that the A64s will work very well with slower memory. Can't remeber where I read this but they specificaly said to use your PC2700 memory unless you just felt like spending your money. The difference at most will be 1% to 2% on most apps.
Speed wise it should be about 60% faster than your Athlon XP 2000 for 32 bit apps that are CPU bound. If you have a 64 bit OS and fully optimized 64 bit apps you should see another 15% to 50% depending on the app for CPU bound apps. See http://forums.gentoo.org/viewtopic.php?t=221045
for a thread that has 32 bit vs. 64 perfomance data.