Time to Upgrade?

We'll need complete specs before we can give any useful advice.
In the meantime: If it ain't broke, do n't fix it   .

I'd say even make sure they don't need to be cleaned out real good and then OC them and you should be fine.

If you are satisfied with the performance you are getting there is no need to upgrade just yet.

Asus P7P55D Pro Motherboard - Intel P55, Socket LGA1156, Core i5

Intel Core i5 750 BX80605I5750 Processor - 2.66GHz, LGA 1156, 8MB L3 Cache, Quad-Core, Lynnfield

Corsair 8GB - PC12800 RAM, DDR3, 1600MHz, Core i5, Dual Channel

Cooler Master V8 CPU Fan - Socket 1366/775/AM2/AM2+,1156, AM3 120mm Fan, Modular Aluminum Fins

OCZ / GameXStream / 700-Watt / ATX / 120mm Fan / SATA Ready / SLI Ready / EPS / Active PFC / Power Supply

Intel X25-M Mainstream 2.5" 80GB SATA II MLC Internal Solid State Drive

And the video cards listed above

You are still really solid and I would stick with what you have as long as it runs what you want the way you want.

I suspect the current cards suffer a fair performance hit with AA enabled. If this is something you can't live with then, and only then, might you want an upgrade.
Be warned, though, it will be expensive, to get any useful performance improvements you'll be looking at the 660Ti/HD7950 as a replacement.
BUT...
Overclocking the graphic cards will certainly help-and it's free.
Oh yes, one last point, that i5 of yours is really, really, screaming out to be pushed harder   . Go on, get the overclocking whip out on that lazy slug as well!