The worse that can happen when you buy a GPU that is bottlenecked by the CPU, is that the card won't operate at it's full potential. When the CPU upgrade happens, though, the GPU gets a performance boost (In comparison to when the CPU was throttling the card.)
Depends on how much you want to spend. The 6670 DDR3 can do 1024X768 with no AA fine, and it costs £50-£55. The GDDR5 model costs £65, but performs 20% better.
Though, if you can, scout around for a 6850. They can be had for £85 sometimes, and perform more than twice as well than the ddr3 6670.