4gb of physical ram - BIOS only shows 2Gb?????? please help

I got a system last year with 4Gb of ram. When I initially installed Vista and then XPsp2 I remember that the system recognized all of the memory. I recently rebuilt my system and now it only shows 2Gb of ram from the BIOS and Windows.

Is there a BIOS setting or something that I screwed up? How do I get my other 2Gb of ram to show back up?

TIA

I've googled all over and not found anything, Tom's Hardware was the closest I found.

The system:
Tyan S2692 (Tempest?) Phoenix Bios V1.02 (9/14/06 is their date)
4 1Gb DD2-667 sticks

I cracked the case last night and pulled the sticks out and checked to make sure there was nothing in the sockets. They appear clear. I also tried a couple of variations on the order the memory went back in and nothing seemed to make any difference. At least until this morning. My machine gave me a BSOD for some reason. That hasnt happened the entire time I've had this machine and I'm pretty abusive toward it in terms of running a lot of stuff on it all at once.

I think I'm just going to go buy a pair of 2Gb sticks and see if they fix this problem. If they don't I'll find a replacement motherboard.