If you can still play all the games you want with it, there's no need for an upgrade. By the time you can't, more powerful cards will be out, or the current existing line of cards will be cheaper.
You'd get buy with nearly every game on just a 5870 and you essentially have two!
Mobo: Asus P8Z68-V-PRO | Processor: Intel Core i7 2600K @ 4.6ghz | CPU Cooler: Zalman CNPS9900MAXR | Videocard: MSI Twin Frozr HD7970
Memory: Patriot 8GB 1600mhz | OS SSD: SanDisk Extreme 240GB SATA | PSU: PC Power & Cooling Silencer 750W
Monitor: Samsung 30" 305T | CD/DVD: Samsung SH-S223F | Keyboard: Razer Tarantula | Mouse: Razer Lachesis
OCC Cool Club Member
GPU Comparison tool on AnAndTech