I would say if you're looking for any worthwhile upgrade, forget the processor and go GPU. To this day I don't get everyone's infatuation with CPU's being the heart of systems, ESPECIALLY for gaming. Throw a single-core Celeron or an i7 3960X paired with the same GPU and watch that framerate... do nothing. Now throw even a step-up GPU in there and you instantly get a marginal, noticeable performance increase. Plus the PCIe standard is awesome because eeeeeverything's compatible with it, unlike CPU sockets being changed every year (I'm sure to help play on the fact that everyone still thinks the CPU does everything, even though that hasn't been the case for over a decade with most tasks users carry out).
Anyway, sorry for the rant but I'd definitely go ahead and grab yourself a new graphics card haha. The 5870's quite a capable card, but if you're just looking for more screaming performance it'll be worth it to go with a 7xxx series or- since you've got an Intel setup anyway- I'd grab a GTX 670 or 680 (depending on your budget). Your CPU is pretty high-end anyway man, no need for a big upgrade there.