I've got problems getting 4 gigs of of OCZ memory running. I can only get it to boot if only one stick is installed at a time (doesn't matter which). Otherwise, it doesn't even POST. Memory is the OCZ2RPR10664GK 4GB PC2-8500 (DDR2-1066). It's not listed in the manual, but a half-dozen other OCZ models are... and I don't think there's a single 4GB listing IIRC.
boot into bios...advanced tab/ advanced chipset / Northbridge Configuration / Bankinterleaving-----set this to AUTO.
save and exit and let the computer reboot.
turn off the computer.
Now insert the sticks on ram and boot up the computer...this should start..it might take afew seconds longer the first time it starts up.
Now enjoy it.
Also make sure that your BIos is updated.