And I wanna add another 2GB card, I wanna buy a kingston one for warranty reasons.
so 1st of all: I should get the same kind of card right? another PC2-6400, DDR2 800MHz so they fit? cause anything faster wouldn't be faster cause it has to worl with another card which is slowr. am I right?
There are four things you need to consider in mixing RAM.
1) What is the voltage requirement of your current and new: Are they equal and can they run at the other RAM's speed?
2) What is the MHZ: Are they equal? 800MHz, 1333MHz... Etc.
3) What type of RAM each is: DDR, DDR2 or DDR3
4) What is the timings (CAS Latency): Are they equal or can they run at the other memory's timings (I.e. 4-4-4-18 vs 5-5-5-24)
Ideally, you want to say all of these are equal which gives you the best chance of success. Otherwise, you need to determine if the one set can run properly at the lower sets specs (which means, the losses / higher timings & lower MHz at the same voltage requirements).
I have totally answered your question but from here you can help make a decision or determine the info "we" are missing to help (timings )