Just wait! You have put up with that '08 Mac this long, no reason to short-change yourself by buying something now. Your best bet is the Quadro 4000 if you do. It takes a single slot width and single PCIe power connector. Offers decent all-around performance and CUDA acceleration for Premiere. Prices are up on the Quadro 4000, which sucks, I bought a few of them last year for about $600 each. Now it's hard to find them for $750 and often to find them under $800 is a blessing.
nVidia is releasing new PCIe v3.0 cards in the next 3 weeks. I would wait. New Mac Pros are coming and there's already hooks in the developer previews of upcoming OSX releases that show support for both the ATI 7900 series as well as the nVidia GTX6xx series. There's a pretty credible rumor going on that we'll be able to use any off-the-shelf nVidia card with OSX 10.8. The drivers are pretty much there as they are now. Latest beta driver release from nVidia supports most GPU functionality with off the shelf (non-Mac) nVidia cards under 10.7. Of course, no EFI support on those cards, so that means no boot screen and no way to select OS volumes to boot from or other such features from the card, but it's a huge step in the right direction. The rumor goes more along the lines of Apple and Intel using the new EFI spec that allows for extended graphics support. It's taking the PC world eons to make the full EFI switch, but nVidia may finally be making the jump and placing a full OpenEFI on their new video cards.
I hate dropping rumors like this because they often turn out untrue... However, I think there's enough out there to substantiate this one, or at least enough to justify mentioning it.
Of course, this is all rumor or hearsay, but given what's showing up in all the developer seeds, there seems to be a lot happening on the hardware front at Apple. Personally, I'm not spending a dime on new computer hardware until I can get my hands on Xeon E5 workstations and Ivy Bridge desktops, as well as a lot of the other new goodies that are coming out in the next 3 to 6 weeks -- new CPUs, GPUs, SSDs and more. The rumor sites are almost totally silent on all this and what is showing up in regards to Apple and I'm not sure why, but I think they're too busy focusing on the notebooks and iDevices to really care, as is most of their membership.