2x2gb or 4gb ddr3 ram?

2x2gb unless that 4GB stick is on the memory list for your mobo. Look up on the mobo's manufacturer site for the tested memories for your mobo. You have 4 RAM slots or 2?

You'll also be missing out on running ram in dual channel if you just get 1x4GB stick... so I'd go with 2x2GB.

If you have a motherboard with only two ram slots, I would go with the single ram stick to allow for future expansion.

For the most part, the two options will cost about the same.

The performance difference between dual or single channel is measurable, but not noticeable in real applications. Think 1-3%

my motherboard is biostar 790gx. 4 ram slots are there. im using 32 bt windows 7. so i dont need future expansions.