Depends on what you consider playable and what image quality sacrifices you're willing to make... I'd get a new video card if it was me the one using that machine (X1950XT, X1950 Pro, or 7900GS), maybe a small CPU upgrade (I'm guessing it's a socket 939) to either a cheap Athlon 64 X2 or opteron, and you'll be ready for any upcoming games.
Thats the thing....how much would I be sacrificing? Would I be able to play it at like maybe 85% of its potential? I am going to upgrade everything in January, but was wondering if I should buy the game now. I understand I wouldnt get the Directx10 effects, but was more concerned about overall game performance and fps.
If you're going to upgrade at any point, I'd do it either now if you have the cash, but if you must wait then I'd upgrade during the christmas sales period (don't know if it'll last until january). Things to look forward around winter (hardware)... only the 8700GTS (G92) and maybe the HD2900Pro (RV670).
And to answer your question, no you won't play anywhere near that because of your card (SM2), bioshock needs at least SM 3.0 to work properly, even though there "patch jobs" for SM2 cards, they are nowhere near the quality of the original.
If we weren't considering that whole X800 fiasco (Mind you this is only an opinion based on my experience with Bioshock), then I guess to run at decent fps (45-50+) constant for a shooter then you'll need to sacrifice the probably around 30-40% in the detail sliders running probably @ 1024x768.