BEZ sent a good link.... and keyshot is one of the best in the market...
Unfortunately, as you might already know the quality depends on GPU...
How ever, if you use Inventor and you did download it from the autodesk student portal you can have for free the Autodesk showcase 2013.
Actually you can get really good results from a fairly cheap machine like mine ;) with the above software.
You can check my renderings may not be the best but I think that can be considered as above average!!!
Don't forget that rendering is a bit of art and any potential post process with photoshop can boost up the quality of your image even more!!!
Check out this: http://grabcad.com/questions/tutorial-_-keyshot-tips
YES me too