XtremeComputing has posted a review on the Sapphire X550 Passive PCI-E 256

The RAM chips used are Hynix 256M-bit, 500MHz (DDR) chips. 4 on each side of the PCB gives 256MB of RAM. While these are rated for only 500MHz, the same series of RAM will run up to 700MHz when the voltage is increased to 2.8V, so I can see a volt-mod coming on.

Sapphire X550 Passive PCI-E 256 Review