If you're running 1680 x 1050, your 5670 should be able to run most games (including Cataclysm) on very high settings. Without testing it I don't know what "Ultra" entails, but you should be able to run near the top of the settings spectrum. This is assuming that you have a decent processor and enough RAM, obviously. If you can't, something is probably wrong. Definitely update the drivers; you should see noticeable improvements, but don't expect night and day.

If you're trying to run 1920 x 1200, you'll only be able to run most games on low details. If this is the resolution you're using, then your results sound pretty standard to me, and you shouldn't expect to improve much no matter what you try. I would recommend against trying to overclock the card if you don't know what you're doing, and can't easily afford a replacement.

If I were you, I'd update your drivers, and see what you think. If you're not happy with the results, or if you're determined to game at 1920, return your card, and buy a 5770. This should set you back about $150, and, in my opinion, is probably the best bang-for-your-buck card out there in the 1920 arena.

Good luck.