I have the M2N-E Motherboard (I am not sure what info to give you so I will give it all to you)
Socket AM2, NVIDIA nForce 570-ULTRA, HT2000 MT/s Dual=Ch. DDR2 800, PCI-E x16 slot, Heat-pipe, etc... So it says PCI-E x16? I have the user manual too and the diagram shows 2 PCI Express x1 slots, 2 PCI slots, a PCI Express x 4 slot, and a PCI Express x16 slot if that helps at all.
Oh yeah I am running a 7900GS graphics card, what is the power required for that and would that tell me if I would have enough power for the 5770?
go get a GTX 460. you wont worry about compatible issue. ATI card is pretty suck from compatible point of view