Also keep in mind that even if you're CPU limited, you should be able to increase more of the graphics options without taking a further hit to performance. Not all games will probably be CPU limited to the same degree. If you plan on upgrading your CPU in the next year or so, then a more powerful card would also immediately stand to benefit from that.
However, an argument could definitely be made for simply saving your money by going with a cheap card and making a bigger jump in the future. I think it's probably worth waiting for next year when AMD has a full line-up out, so we have more price competition between AMD and Nvidia.