Heya,
At really high resolution, you really won't have that big of a deal with AA standards. 4x is great at high resolution. Try 1600+ resolutions with 4x and 8x AA and screen shot it. See if you can really tell a real difference. Especially at the 1900+ resolutions. AA was really something that made drastic changes at lower end resolutions (1024x768 for example), but at the upper side of things, you'll just not see that difference as much.
BioShock will run at absolutely max everything on a modern card. I mean, the 8800gt can run it like that and it's a 2~2.5 year old card now. The HD 4850 will do it without even breaking a sweat.
Very best,