The problem is that your memory won't run in Dual-Channel mode if you stick a separate 1GB DIMM in there.

If you want 1.5GB, I'd suggest 1x512MB DIMM and 2x256MB DIMMs, BUT that board only has 3 slots for RAM.

I suspect that 2x512MB of RAM in dual channel will perform better than 1.5GB in single channel. You may also want to consider 2x1GB DIMMs instead.