Play Crysis 2 on a iMac via External Graphic Card

So I'm new to Mac and I got an iMac 21,
I'm running Win 7 and uses it mostly to play games..
I'm currently playing Crysis without any big problems however Crysis 2 is due to be dropped soon and I dont think it will run on my iMac Mid 2010 21.5´

I checked if it's possible to run Crysis 2 on "Can You Run It?" and it says I cant due to my Graphic Card

Minimum: NVIDIA GeForce 8800 GT 512 MB of memory or higher; ATI/AMD Radeon HD 3850 512Mb RAM or higher
You Have: ATI Radeon HD 4670

Is it possible to upgrade my graphic card, preferably to Nvidia if not Nvidia to ATI Radeon (higher than ATI R 4670)
And if it's not possible to upgrade my Graphic card can I use an External Graphic Card?

You can't upgrade the graphics card in your iMac as it's integrated (soldered) on the logic board. Actually, surface mounted.

I can't answer your question about an external graphics card though, perhaps someone here who has used one can answer.

Seems like this question as been asked before on Apples discussion page.

Doesn't seem very promising. >_<"

External video cards are severely inhibited by the bandwidth of the external bus. An internal, PCI-e card is going to be an order of magnitude faster than something attached via the comparatively feeble USB bus.

What external cards are useful for is massive multi-monitor setups, but that's about it.