Hi, I have on my system 2 gb of Mushkin HP2-8500 clocked at 1066mhz with 4-4-4-12 and 2.19v
I have an additional 1gb kingston kvr800d2n5
Should I put this too ? I would gain some performance or I will just loose ? Can i even clock it at the same as my other memory?
You will gain performance if you are doing something that needs the extra memory. If you are not, then your performance will drop slightly.
First important thing with memory is "do you have enough?" If you don't, then more memory, no matter what the speed or configuration, is better. As even the slowest, cheapest, most mis-matched memory you can get to run is still 10 times faster than having to go to the hard drive if you are running low on total memory available.
If you have enough memory, then the concern changes to timings, matched sets, dual channel mode, bus speed, things of that nature to tweak the speed of the memory. But this is a much, much, much minor thing compared to your PC running short on memory.