depends on the purpose
for gaming:
the best 64-bit CPU is the FX-57 (they have that out now) at about $1300
the best dual core 64-bit CPU is the Athlon 64 X2 4800+ (even without 64-bit) at about $1050
for multimedia and mathematics:
the best 64-bit CPU would be an Opteron 852 at about $1500-$1700 (if that's in your price range) or a Xeon 3.6GHZ at about $650
the best dual core and 64-bit CPU is:
the Opteron 852 or Xeon 3.6.
if you're talking Athlon 64 vs Pentium 4, the Pentium 4 is a better math CPU, and the Athlon 64 is better for DirectX and games