An Intel C2D E6600.
But of course, that would require an entire switch of CPU/mobo/RAM, so if a serious upgrade isn't quite the plan, then you could look into basically any of the s939 A64 X2s.
Best bang for buck is almost always the lowest end of the line, which is the X2 3800+ (or Opteron 165, but AFAIK, it's still overpriced right now).
Now if you don't OC, then you need at least the X2 4400+ to match your 3700+...assuming it's a s939 3700+.
If it's s754, well then i might suggest considering the C2D route, depending on what other components you can re-use.