QUOTE(breekachu @ May 26 2015, 01:11 AM)
ahhh nice point you got there sir
Q: can i use different type of ram? like Gskill Aegis 8GB 1600mhz DDR3 + TridentX DDR3 + RipjawsX XL DDR3 . as long its DDR3 its okay right?

Generally yes, since DDR3 is at its end of development, so probably no more new variants will come out (except DDR3L). So that means any 8GB you can find now will be the same as 8GB you can find in future (the "same" refers to the chips specifications).
But still, you should take note on 2 things, latency and speed. Sometimes different latency and speed RAM can't work together.
And no, you can only use OEM key once on 1 device.
For retails key, you can transfer it quite a few times by contact Microsoft, but still only one at a time.