looking at your video card you'd definately see a big gain in fps if you opted for a model with more horse power, what I wouldn't recommend is upgrading the video card unless you can get one thats at least 50% faster than your current one, also be aware top end cards wouldn't be justified with a dual core... the data throughput would bottleneck long before a top end cards (Ati 5850/5870/5890 or Nvidia 285/295/470/480) would get busy although a useful stop-gap if you going quad+ at a later date.
I'd say a midrange card would be the best match for a dual core, something like a ATi 4850 (dx10) or 5770 (dx11) or for nvidia flavours the 260 or 275 (both dx10), check out the numerous benchmark sites on the web for more details.
Also worth...