I just tried putting the Mushkin into Slot #1 and leaving out the stock ram completely. Boots up fine, but only shows 256mb RAM total.

So I can't seem to get more than 512mb, and that's with both in at the same time, with the stock 256mb stick in Slot #1 and 1GB Mushkin in Slot #2.

My system is capable of running up to 2GB of RAM total, according to specs.