the fastest CPU depends on what applications you run.
for gaming, the Athlon 64 FX-57
for gaming + multimedia, the Athlon 64 X2 4800+
for raw CPU power, and multitasking in 8 threads, the Opteron 852
and like Dark_Sniper* said, it also depends on your budget. you might not be able to pay for really high-end CPU's. especially the 852, which is about $1500 US on its own
the fastest CPU you can get in your motherboard (Socket A) depends on how high the FSB goes in it. you'd be lucky if you have 400, but I'd say you probabbly have 266 and maybe 333.
the highest-end Athlon XP is the 3200+ (400 FSB)
the highest end Sempron in Socket A, I think is the 3100+ (that might be only socket 754 though)
there is the 2800+ with a 333FSB