PowerEdge 800 - Memory Question

There are four slots for memory on the PE800

DDR2 400MHz is still available ... you just have to look around (unbuffered ECC is not common):
http://www.provantage.com/axiom-memory-solutions-ax11890789-2~7AXM947F.htm

That would be preferred, but if you ended up getting the 533MHz, it will work, but all the memory will run at the slowest common speed ... 400MHz.

It has Four slots.

Currently 2 slot being used with 512MB DDR2 400 MHZ RAM

Total 1GB. I would like to add another 2GB. Only DDR2 533 MHZ RAM available in market.

Is it ok to mix both DDR2 400 & DDR2 533 RAM

Like Bank 1 contains DDR2 400  Bank 2 contains DDR2 533 Total 3GB RAM

Right now using Samsung (2) 512mb pc2-5300  and (2) 512mb pc2-4200   in my PE800.  All are non-ECC, unbuffered and non-registered.