SLI is NVIDIA's multi-GPU technology... while CrossfireX is ATI's tech for multi-GPU usage. You can use your ATI card, but you won't be able to upgrade to CrossfireX.
The case includes a 450 watt supply
PSUs that come with cases can sometimes spell impending problems... call up Tigerdirect and ask them for the amperages, make, and model of the PSU included.
I would go with the Corsair one... http://www.newegg.ca...N82E16817139008
I know it has a 8 pin and 20+4 pin connector (all ATX PSUs should have those). It also has very high max amperages for a 400W PSU and delivers clean power.
So you are not going to get another CPU... you can stick with your old one for now and upgrade later down the road.