They'll have to be HHHL - Half Height Half Length - or smaller. I'm not sure any are other than the bottom of the ranges - a quick search found a GeForce 710 in that size, but that's fairly puny. I did also find a K620 in low profile, but I suspect it'll be too long; try looking at your favourite supplier for "low profile" to start with, the best I can see is a single slot K1200 from Leadtek, but again length could be a problem.