Good Single Slot power-through-PCIE GPUs

-to fill who ever reads this in- all I'm trying to build a system that is just stupid powerful for no money at all. I'm working with this rig I dug up and, so far, have turned it into a value performance monster;

*Xeon X5450 3.0ghz

*6gb ddr3 1333 (soon to be 8 if I can find that cursed 2gb stick around the apartment. )

*Dell OptiPlex 780 board/case (modded to take my 771 CPU)

*250 , 7200rpm WD

*Dell OEM 305 Watt PSU

I've been looking at a Firepro series card from a certain vendor, but some guy snaked a 20 dollar V5900 out from under me (seller had no idea what he was holding on to obviously), but now i'm only stuck with a Radeon HD 6670 as an option. Not to say it'd be a problem, but being that the OptiPlex board is a BTX form factor, I am limited to single slot solutions. You guys have any good powerful suggestions?

I do have headroom for a large cooler above, but the actual IO can only be single slot if you do have any larger cooled suggestions.

(good note, i'd like the card to be below 100$, just a note)