there are no windows 7 drivers* for the gx620 either. but i am writing this post on an optiplex gx620 running windows 7 with no issues.
and i got it wrong way round earlier, the gx520 has two DIMM slots, 533Mhz, max 2GB
*from dell, anyway http://www.dell.com/support/drivers/us/en/19/Product/optiplex-gx620