Have you tried removing the RAM to see what is causing the issue? First step would be to try to isolate the problem to a specific stick of RAM or the HD.
I have a Quad, and funnily enough, also added a 300GB Maxtor drive and 4GB of RAM from OWC and have no problems with sleeping or anything else. Sounds like you got a bad part. OWC will take care of you if you can find out what is causing it.