I've heard mixed results on anyting above 2GB of RAM. If you do have over 2GB of RAM, there is a virtual switch that must be engaged in order to allow inventor is utilize the additional RAM. Autodesk forums will have more information on this particular issue.
And yes, graphics card & driver are extremely important. Get the list of certified cards/drivers from Autodesk, and hand that to your IT department. Fight tooth & nail to get the biggest baddest one on that list.