The gang at the [H] take a look at the differences between running Bioshock under Vista & DX10, & running it under WinXP & DX9. The verdict? Meh.

"Overall there is no compelling reason to upgrade to Windows Vista to run the DX10 codepath in BioShock at this specific time. There aren’t any major visible features that would compel anyone to make that switch, especially with the lack of AA support in DX10."