Can I mix DDR2 533 with DDR2 800 ram in the same system?

"When different speed RAM modules are used, all of the RAM should clock to the speed of the slowest module. Many times, I've found that mixing RAM speeds sometimes causes system instability. I've had plenty of cases where RAM sticks that tested just fine alone wouldn't play well together. So I strongly recommend that all of the RAM installed in your computer be of the same speed.

It depends on a lot of differents aspect, but as long your Asus motherboard supports DR2-533 / DDR2-667 / DDR2-800, I think you can try without causing troubles to your mobo."

It should work. Make sure you set your BIOS settings to automatically detect your RAM timings/frequencies.