Selecting a Server Graphics Card for Small VDI Deployment

Folks:

We have a server running Server 2012 which is hosting 6-10 virtual desktops.  (The server is a Dell PowerEdge T620 with two E5-2650 processors, 32G ram, two RAID arrays, 10k drives.)  Even when only a few desktops are running, performance is slow. We are considering adding a graphics card.  

Does anyone have thoughts about whether it will improve performance?

And if so, any thoughts about either good card choices or what features/capacities would be useful?

Thanks!

Micah