I have a little misunderstanding.
Now i`m running my system (W7/64bit) on 2GB DDR2 667Mhz (no name) and I'm planning to buy 4GB Kingston HyperX 1066. My question is: Should i keep the 2Gb too, or shoul i let only the 4Gb HyperX ?
Take it out. No need for 6 GB, and theres a good chance your motherboard will detect 667 mhz and set that as the speed for all the RAM to operate at.