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 P6TD Deluxe | Processor: Intel Core i7 920 @ 4ghz | CPU Cooler: Zalman CNPS9900MAXR | Videocard: MSI Twin Frozr HD7950
Memory: Patriot 12GB 1600mhz | OS SSD: SanDisk Extreme 240GB SATA | PSU: PC Power & Cooling Silencer 750W
Monitor: *Debranded* HP 24" 2408H | CD/DVD: Samsung SH-S223F | Keyboard: Razer Tarantula | Mouse: Razer Lachesis
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GPU Comparison tool on AnAndTech