This really depends on the spec of the computer in question.
I haven't tested for about 5 years, but, I noticed that putting a cheap graphics card in to a machine that was using integrated graphics made general activities much faster - I can only guess that offloading stuff from the chipset increases overall performance.
That being said, from what you are saying - as you already have a graphics card, the first thing I would do is just try updating your drivers... but, I see things like that from time to time... but, a GTX 275 should be enough, so, I am inclined to say there may be another weak link in performance - perhaps your CPU just can't cope with the virtualisation - remember that graphics in the guest are most likely being emulated/run on the CPU.
The GPU only needs to support DirectX 9 in order to be able to run Aero. – Bogdacutu – 2013-07-11T13:24:17.913