Question on "Mixing" Memory

Last May, I purchased this set of RAM for my first build. I've had no problems thusfar, but I'm thinking of upgrading. Due to a mixup on a recent Newegg order, I have $15 credit, so I'm thinking of buying a new set. My motherboard, the Gigabyte GA-EP35-DS3L, has 4 dual channel slots.

My question is, is it safe to put, say, a 2x2GB set of DC RAM in the other 2 slots? I'm guessing they would need to be similar voltages and latencies, so I'm thinking of buying this set: http://www.newegg.com/Product/Produc...82E16820231122

What is my best course of action here? Will I be okay if I put the set posted in here, or should I just swap out the set I have for a better one? If that's the case, does someone have a better 2 x 2GB suggestion?

Thanks for any info, much appreciated.

EDIT: I am still running vista 32-bit, but I plan on moving to 64 very soon.

EDIT 2: Or, would it be worth it to upgrade to 1066 memory and ditch the 800? It's not that much more.. not sure what would be better 6GB @ 800 or 4 @ 1066..