I would Say 8800 gtx> It does fit but you have to disconnect the 1394 wire to get it in.Highest processor to put in the a1730n is amd 64x2 5400+ 2.8 Ghz. That is what I did with this pc. I played crysis with decent frame rates at 1920 x 1200 resolution,but not great. 3d benchmark got a score of 9,880,with a little overclocking.