1. Swap your OLD RAM spaces - 1 GB to 512 slot, and vice versa. If that powers on OK, then the RAM chips are compatible with both RAM slots. If it doesn't, then it looks like you need to keep the 512 in there.
2. Keep the 512 stick in old slot, and replace the 1 GB OLD with NEW. If it powers up OK, then your NEW RAM is OK too.
If both 1 and 2 work, then you shouldn't have a problem. If not, pull out the 512 stick, and put the OLD 1 GB in its spot just by itself. If that doesn't work, then it looks like you need to keep the 512 in the old slot after all.