Running on x64-bit with 4GB right now and I don't have this problem, so to see someone else have this issue really is horrible. Try this:
Go to:
Start -> Search (or Run) -> msconfig
Go to Boot tab -> Advanced Options...
Make sure "Maximum Memory" is unchecked. If it is checked off with less than 4030MB this could be the problem.
Confirm Vista is 64-bit: Control Panel -> System
Under System type it should confirm: 64-bit Operating system
If both of these are good this is surely something in the BIOS that is not set right. Look for a Memory Hole or Memory Remapping option and set to disable or software instead of hardware.