The 5850 is more powerful than that 6850, considering the 6850 is meant to replace the 5830 (6870 replaces the 5850), so I should hope you can use 8xMSAA with your 5850 and still achieve good framerates, because I can as well with my 6870. 6850 will be a bottleneck at 8xMSAA, unfortunately. 4xMSAA MIGHT be pushing it with the 6850, though I don't know.
The 6850 will be your bottleneck at 8x MSAA. You could go for a 6870 or a GTX 460 1GB, both would do 8xMSAA @ 1920x1080 on Ultra w/ "good" or "fair" shadows.