i got a question about ram, i have:

slot1: 128 / slot2: 128 / slot3: 256 / slot4: nothing

could i get another 256 and put it in slot 4?

Yes. Another aspect depends on which processor and motherboard that you have. If it's a Pentium 4 with a 533 or 800 MHz front-side bus, pairs are best, anyway. If that's the case, you are taking a performance hit right now by not having 2 pairs.

ok im not really following, so if I have 256 256 128 128 then i would not be getting as good performance as if I did just 256 256?