It's an older socket, so the processors that might be available won't cost too much, you have two ways to go as I see it.
Phenom quad core: 4 cores / 2.3GHz(cpu) / 1.8GHz(fsb)
This should run NR2003 just fine, has higher FSB and 2 more cores, plus it's thermal output is less and will run cooler and probably live longer.
It would also be better for modern games that can utilize multiple cores.
Athlon 64 X2 : 2 cores / 3.2GHz(cpu) / 1.0GHz(fsb)
This one is clocked almost 1GHz faster than the quad core, but it's only 2 cores and you lose 800MHz on the FSB, and it's thermal rating is quite a bit higher meaning that it will be harder to keep cool under load.
Since you are upgrading the CPU but not the mobo I am assuming you have a small upgrade budget, unless you have a case that moves a lot of air, and a great heatsink/fan for the cpu, I would go with the quad-core phenom because I would be worried that you might have to spend more money later trying to keep that dual-core 125W cool enough to run under load.
I am not very familiar with the AMD's, in the early 2000's I built 4 computers using AMD processors and was never happy with their performance compared to Intel so I quit using them and I am judging the thermal output of these AMD procs based on my experience with Intel/Xeon processors that I have used and seen a huge difference the cooling needs of a CPU rated over 100W vs those ~80W or less.
