I just went trough this, and the thing now is to get a Vid Card that supports Pixel Shader 3.0. Because all new games have this shader, so if you plan to play new games you better get this. Games like F.E.A.R., DoomIII, CoD2, BF2142, etc. will ask for this Shader to actually let you install the game.
Now the lack of AGP or PCIx slot is really a problem, because to get the best performance you gotta have this. In my case I still use AGP, and got a GeForce 6800XT
. Maybe look at your sis PC and see if it has AGP or PCIx port to get a good Video Card. Here
is a explanation of the Pixel Shader.
According to what I reserched, all Vid Cards above GeForce 6200, and ATI X1000 series will support this Shader.
Hope it helps