Like I said in dude's other thread, I'm using a GX 620 right now.
I've tried constantly to find ways to fit a dual slot card in it, but to no avail. The chassis is BTX, so everything is reversed, the PCI-E slot is at the left most bracket if you lay the computer down sideways, and the damn Dell CPU fan is so huge that it would block a dual slot cooler anyway.
Low profile single-slot cards are really the only option. I'm stuck with a Radeon HD 4550 in my computer right now because I couldn't afford any other single slot card that didn't suck as much as it did (though I did get it for only $25, so I guess I can't really complain)--this is the main reason I started that AM3 build thread; to find something better, because I hate being limited so much by the GX 620.
I doubt he would like a 4550, so I'm saying over and over to go with the 5770!