PC4200 is 533 MHz, like Emily my Dimension E310,whuch only takes two, and I only have one so I've got 2x 512MB PC5300/667 at the moment as it only has two slots (which I posted in my thread at about 3 am. I believe mine might max at 4 GB but that's an expensive proposition unless somebody on CL has it used cheap/wants to trade.
I'm thinking you spent a tank of diesel for that RAM.