if you can afford it (dont know US prices), the 8800GTS 320mb is great for bioshock, dont get the 640mb unless you plan on running 1920 widescreen (24 inch monitor, i think.)
for PSU grab at least 500W, ideally 600, from a quality brand.
dont go for SLI, for reasons zombieloffe said, and although you probably wont be running DX10, its always good to have the option. but dx10 cards are already outdated, thanks to microsofts dx10.1.
EDIT: i'd tell you to consider a new CPU, because a pentium 4 would have trouble with bioshock (if it'd even run at all!). i'm talking painstakingly low graphics here. but that would mean a new motherboard ($70-80), new RAM (not much more, considering you're buying most of it already, and DDR2 is cheaper these days), possibly a SATA HDD ($60-80, depending on size).
because of the age of the cpu, just check if its actually PCI-express. if it aint, you need all those things for a new graphics card.