Shuttle refuses to boot with RAM upgrade

[Stringy]

I bought 2 x 1GB pc3200 Corsair XMS off the members market to go in my ST20G5 in place of 2x512MB Corsair Value. When I put the 2 sticks in the computer refuses to POST, and doesn't throw up any error beeps or anything. However, each stick individually works fine, and I know both slots work as the 2x512 worked when I put them back in.

Anyone got any ideas?

edit: It'll even boot with 1x1GB + 1x512MB for 1.5GB total

 

[Stringy]

Well I played with the voltages to no avail Any other suggestions? Is this something that updating the bios might cover? Will go floppy drive hunting if so.

 

[Stringy]

BIOS flash achieved nothing...

So, 1.5GB in single channel, or 1GB in dual?

edit: or have I misunderstood the concept and 2 unmatched sticks can still be dual??
editedit: nope, working in single chan, so the original question stands