In that case, I'd guess that either the RIFT bottle doesn't contain all of the necessary components, or that the LegacyRenderer option wasn't actually applied. That Radeon HD 6750M ought to be good enough to run the game.
Go to Crossover's Configure>Manage Bottles menu. You should just have one "RIFT" bottle, but if you've got more than one you'll want to find the "right" one and delete the others (you can delete bottles using the "-" key in Manage Bottles). A properly-installed RIFT bottle should have the following components:
Crossover HTML Engine
DirectX9
Microsoft Visual C++ 2005 Redistributable
Microsoft Visual C++ 2008 Redistributable
Microsoft Visual C++ 2008 Redistributable -x86 9.0.210022
RIFT
RIFT
TrueType font: Andale mono
(and then a series of other true type fonts)
If you've got just one RIFT bottle, please confirm that it contains all of the above components, and that the "rift.cfg" file has "LegacyRenderer" set to "True".
If your RIFT bottle does not have all the proper components, you can install what's missing into the bottle using the general principles outlined, here:
http://www.codeweavers.com/support/wiki/mac/mactutorial/unsupported_deps