run into? As long as your Power Supply is sufficent, and you have the correct AGP slot, I don't see any problem with you switching.
I currently have a 875p Intel 3.2 I run a ATI 9800 AGP and would like to switch to Superclock e-GeForce 7800 GS CO w/ EVGA Top Plate AGP. Is there any reason I cannot do this? Is there any problems I would run into?