Can I run a config of 1 GB DDR2 on two channels (dual) and in the othe two slots 512 each or does that also ha

I have two 1 GB sticks on my MB now in dual channel setup, and i have two open lots that i would like to tick with two 512 MB sticks to give me a total of 3 GB, the most XP recognizes. I just want to make sure there are no issues with one channel having 1 GB sticks of PC2-4200 and the other two slots having 2 512 GB sticks.