I used to have a low profile gaming rig (about 1 year ago). Here is a quick rundown of what I've tested in it:
First try was a MX440se. That was junk.
I had a PowerColor 9100 low profile card. Worked great up through BF1942, Quake3, UT2k3, Call of Duty, Neverwinter Nights, etc., but the newer games on the market don't run so well.
I tested a 9600se lowprofile card, but wasn't impressed. Obviously it was a better card, but I wanted more than it could offer as an upgrade from my 9100.
In the end I broke down and bought a mATX non-lowprofile case. It's still small, but allows me to run a 9800 Pro.

That allowed me to upgrade to a Mobile 2500+ AMD cpu, and overclock it to 2.5ghz. I've modded the case quite a bit, added lots of fans, etc. It's still small, and pretty damn fast.
Low Profile is awesome if you have space issues, which I do. I just had to make the decision of gaming speed vs. space. I was able to get a normal matx case to work in my space, so I went for it.
As a side possibility. Depending on which low profile case you have. It *may* be possible to setup an AGP slot riser card and get a full size video card to work. Good luck though, I think it will have to cover all your PCI slots, and then some.