If I buy this can I use external graphic cards?

As far as I know, modern graphic cards are much faster than Thunderbolt offers, Thunderbolt offers speeds for GPUs that are some years old already, thus you might not get any performance boost after all.

 

It only gives a max of an 8x slot, most graphic cards are x16 slots, i would think any 8x slot cards would perform worse than the integrated one you already use

 

8x is fine for graphics cards... NVIDIA used to have SLI setup so that if you had two on one motherboard it would make them run at 8x instead because the motherboards couldn't support 2x16 lanes.

However, the problem with this PCIe chassis is that PCIe cards need to be compatible with Thunderbolt technology, which are either none or maybe one.

For a current listing of compatible cards visit http://eshop.macsales.com/tech_center/Thunderbolt/PCIe_Compatibility/

 

Probably not the right solution if you need high end graphics.

 

and where would you get the osx drivers for the graphics card?