PC won't POST when I have both Ram Sticks installed (each work when installed individually)

Hi! I'm currently trying to get the most use out of an oldish machine.

OS: Win7 64bit
MoBo: Asus P5KPL-AM EPU
CPU: Intel Core 2 Duo E8200, 2.67 GHz

The MoBo has 2 slots for DDR2 Sticks (both colored yellow). I checked the manual, and there doesn't seem to be any needed order when installing. It allows up to 4GB.

When I bought this old machine, it already had a 2GB stick installed, a Kingston 800/PC2-6400 KVR800D2N6/2G

I was able to find another 2GB stick, a Samsung M378T5663EH3-CF7 800

I don't have the packaging for both sticks, so I had to go by research based on serial#s, but as far as I can see, both sticks have the same speed (don't know how to check for CAS Latency... does that matter?)

When I check using CPU-Z, it's weird since although I only have 1 stick installed, CPU-Z says that I have 4GB, and the current stick appears for both DIMM1 and DIMM2 (also, "both" are running at 400MHz)

Anyway, when I put both sticks in, my PC wouldn't go to POST. I tried each stick individually on DIMM1, and the system loaded without any problems... I haven't checked yet if putting a single stick on DIMM2 will work, but I assume that it will (if the slot is really busted, then wouldn't the system not be able to detect anything there, so it would still boot normally, right?)

Any idea what I can do?

Thanks!