Hi there ... as a matter of fact, at the end of the last MNM show, a group of us went to Jason's and did just that, and ... I caught a bit of it on video and added it to the end of the video. You can see any CE2 videos in the MSR CE2 Video Library, which will grow as time passes, and the more fun we have.
Pretty well, I had to upgrade a few things such as ram and video card of course, but I can play it on low settings with little or no lag, still looks awesome I think even on low settings. Maybe I will get a better video card in the future to see what it looks like on the highest settings, only using an nvidia 6200 right now.
Both those cards you linked me to are good. Its up to you if you want to go the 4890 route. Thats what I have, its fantastic, 4890 will give you about a extra 15-20% performance on top of that 4870 but not much more unless if you OC.
ATI Stream Processors are much more than Nvidia, but the difference is that Nvidias have more power, eg Nvidia has much less Stream Processors but they each have their own clockspeed as for ATI has a giant clockspeed for many many Stream Processors to share. Its just a different way of getting the job done. Tbh, both are fantastic, however with the Nvidia issues right now Id reccomend ATI, also because every single one of my ATI GPUs still work, and ive had 4 nvidias that all have died. But that could just be my experiance. But I do know that ATI handles high resolutions well and AA. Also ATI is cheap and good, So yea Id reccomend a GTX 260 c216 or a ATI 4870. Since your leaning ATI go for the 4870 it will save you quite a bit of money and its a great card.