Hello you can find info here:

http://www.mercury-pc.com/product-detail.php?link=p-mainboards&subtitle=Mainboard&productid=677

The mainboard supports:

2 x 240-pin DDR2 DIMM socket support up to 2 GB DDR2 533
Maximum: 2GB

533 mhz is probably limited speed by the CPU and the FSB settings.
So you have to try with another type of ram module, the 533 module, because 800 mhz is probably to fast for this mainboard.

Good luck