Changing a graphics card?

I have an iMac with an ATI Radeon HD 4670 graphics card. This is a dedicated graphics card using an internal PCIe slot. Despite being hard to reach, this SHOULD mean it's swappable with a different card, am I right? Driver issues aside, it theoretically should be possible. Only thing is, I don't know anyone who's done it, or where I'd even begin to buy one.

Any thoughts?

James!

PS, I want to play SC2 on 'ultra' everything! BOOYAH!

Hardware Overview:

Model Name: iMac

Model Identifier: iMac10,1

Processor Name: Intel Core 2 Duo

Processor Speed: 3.33 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 6 MB

Memory: 4 GB

Bus Speed: 1.33 GHz

Boot ROM Version: IM101.00CC.B00

SMC Version (system): 1.52f9

Serial Number (system): ---------------

Hardware UUID: F7DDCCFB-539A-51BC-B31F-1FDCAEB84164