I've seen some stunning examples of rendered grass created with Vray using grass meshes as proxy objects and then scattering those grass proxy objects around. I tried a similar setup with mental ray and thought I'd share my findings.
I'll begin by showing the render results from using scattered proxy grass objects (click image to enlarge):
This test really started from a discussion at Vizdepot.com about creating realistic grass in mental ray & 3dsmax, here's a link to that discussion for those that are interested: Vizdepot Link
You can read through that thread for more details on the process but in a nutshell I created some grass patches using the grass-O-matic plugin. I then converted that grass into an editable poly mesh and wrapped it inside a mental ray proxy object. I then scatter it manually or with the help of a scatter plugin written by Peter Watje. Both plugins are available at maxplugins.de
One of the final grass patches that I used is a 5'x5' patch and is quite heavy with geometry at 992,000 tris! Yes, it could be optimized further (always can) but then again just imagine how many actual grass blades are in a 5'x5' area of lush, thick grass. That being said, I can still easily render several thousand instanced copies of this thick grass proxy object, even though I'm still working on a 32bit Vista machine.
I've uploaded my proxy'fied grass mesh for you to try/use if you want: