According to this, older Mac Pros cannot handle 8800 GT... My question might be naive but I still want to know: why? Is it because older Mac Pros do not have PCI-E slots? Is it because Mac Pros have different interface standards for videocards that those videocards are made to fit them? Say if I buy a Mac Pro, can I use 8800 GT purchased from elsewhere (or even 2 of them for SLI)? Thanks!