i have a P5P43TD PRO motherboard and i want to buy a RAM with highest performance.
can i buy these?
Corsair 6GB Triple Channel DDR3 2000
or
Corsair 6GB Triple Channel DDR3 1600
moreover, does works my old RAM (2GB Geil DDR3) with this new RAM?
This board is a dual channel one. You will be able to use DIMMs from a triple channel kit, but you won't get triple channel operation. (In fact, you may need to use DIMMs in matched pairs.)
If you want to be assured of compatibility, use RAM from the QVL (qualified vendors list) on the page you've linked to. The main problems with the QVL are: it tends to be out of date, and there are all sorts of compatible RAM that's not on the list because Asus didn't test it.
You may be able to mix old and new RAM, but I think that you'd usually have to set the RAM all to the same voltage, frequency, and timings, manually in the BIOS settings. You'd be limited by the slowest RAM.
Quote