which graphics card to upgrade too?

[MadDog56]

GTS 250 is high end. You also forgot the fact that the OP only has a 1280 x 1024 monitor, therefore the CPU does a lot more work (compared to 1680 x 1050 and above).

The GTS 250 is a mainstream product, high end is the 285, 295 and the 295 x2.

Read through that CPU VS GPU article a bit and take a look at the benchmarks. For instance with a 9800GTX the dual core at 2.4ghz is about 9 frames per second lower than a processor running near 3ghz.

If you scroll down to the 9800GTX with the core 2 quad q6600 you'll see that the increase in clock speed from 2.4ghz to 3.2ghz actually end up losing them a tenth of a frame....so at this resolution the CPU isn't the limiting factor.