I have the computer put into an Antec 900 case. With that one I upgraded the power supply which you will need to do if you do a bigger card. I put a 450 watt MSI Turbostream in mine and an 8800gt OC from BFG. I was running 4 gb of DDR2 and an AMD Athlon x2 5200+ also. Its been a while and I am unsure about todays numbering system and the cards require a lot less power then the 8800gt. I am upgrading my GFX card as well and trying to find a suitable one near or better then the 8800gt that a 500 watt supply can run.
You should start your own thread.