Mixing memory

I am using Biostar TA780G motherboard which currently has 2GB via

Corsair XMS2 TWIN2X2048-6400C4 2GB 2X1GB PC2-6400 DDR2-800 CL 4-4-4-12 240PIN Dual Channel Memory (http://www.ncix.com/products/?sku=18...acture=Corsair)

I am going to rebuild a bunch of hardware and want to upgrade the memory to 6GB. Memory prices sure aren't very cheap but newegg has this guy for a pretty good price.

CORSAIR XMS2 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Desktop Memory Model TWIN2X4096-8500C5C G (http://www.newegg.ca/Product/Product...82E16820145247)

So there are few questions

• I am pretty sure I can run the 2banks at separate speeds, however, the motherboard specs indicates that it only supports 1066 with an AM2+ processor (going to use up my X2 5400+ first). Is it a safe assumption that the memory should "downgrade" and run fine at DDR2-800 (which I am happy to run at)?
  
• The second question was going to be voltage but I just checked the newegg site for the 2x1GB I already have and it indicates 2.1v ... so I guess I don't have to ask about running different voltages on different channels.
  
• Any other concerns that I should have?
  
• Any other recommendations?

on the DDR2 based AMD cpu's all the ram will default down to DDR2 800 when you use more then 2 dimm slots. From my bit of reading latly seems on the AM3/DDR3 platforms there is no downclocking when using all dimms with ram.

Your best to use the same models of ram when doing more then 2 moduals that way latency's & voltages are matched up.

I'd say buy 2 new matching sets & transfer the 2 gig kit to another system that might only need 2 gigs or ram, or sell them.