Does not power down! odd reason...

Greetings all! I just build a new system (nothing fancy) - MSI K9N2 SLI Platinum board running a 7750 BE along with 2x1gb Crucial Ballistix memory (also 2x 8800GT, TX650W corsair psu) all updated drivers, etc. The odd thing happening is as follows:

When the memory is set to 1066 the system will not power down. Windows shuts off, my monitor goes on stand-by, the hard drives shut off, etc - but the system fans are going, and all the mobo lights on the inside stay on. I have to hold the power button on the front for 4 seconds for it to shut off. When the memory is changed to 800, the system does power down as it should. I have tested it many times with both memory settings and the results are predictable: 1066=does not power down, 800=powers down. Regardless of the settings, the system, while running, is stable, great - but the powering down situation is the only thing that seems to be an issue... Oh, and i did update to bios v3.3 and the problem persists. Thanks for reading!

A few additional questions:
1) What OS are you running ? XP or Vista ?
2) What other add in cards are in the system beyond the graphics card ?