My new motherboard - Asus's excellent Crosshair - came in today. I hooked it up, wiped my hard drive, and installed Vista 64. Then I plopped in my new sticks of RAM, and it booted flawlessly. I'm now gaming in 64-bit with 4GB of RAM. Yay!
Seems like it was entirely the MSI motherboard that was keeping my from using the extra 2 sticks of RAM. That's OK, though. MSI wiped themselves off my hardware approval list, and Asus is back on. (Had some issues almost a decade ago with their motherboards and cards.)