All DDR1 Ram should theoretically be 2.5v, but in practive most manufacturers vary with their recommended voltage. I have seen more DIMMs rated for 2.7v than anything else, alot of people recommend 2.9v or higher as a matter of course. Unless you go mental (i'm talking voltages your motherboard probably wont even support) then you are not likely to damage RAM with overvoltage like CPUs
The DDR3200 standard itself has no differences in voltage specifications than the DDR2100/2700 standards.
Even if you dont overclock and run the 400MHz ram at 333MHz, then you should be able to run tighter timings.
DRAM doesnt have a set frequency for stable operation, it has a max frequency, and can run fine at speeds below that.
What about putting 2, 256 banks of 2100 into a system that has 2, 256 banks of 3200? My question is really, is it better to have a mix of 2100 and 3200 with a higher 1 GB RAM than having only the 512MB RAM of 3200?