Hi everyone!
I wanted to ask you. I have four slots for RAM memory on my motherboard. 2 slots are taken by Dual Channel 256 Mb DDR SDRAM and 1 by Single Channel 512 Mb. I wanted to buy a single channel 1 Gb RAM, but somewhere on the internet I read, that if two of your slots are taken by dual channels, the other two must be filled with the SAME dual channels as well, otherwise they will ALL work on their lowest memory. Is it really like that or it doesn't matter how I combine my RAM?

Plain simple... Yes! The memory sticks MUST be the same (speed, brand, etc) for Dual-Channel to work properly and dependable. Otherwise you are calling for a lot of troubles.
RAM is very inexpensive now-a-days; best bet is to discard the small 256Mb sticks and discard also the 512MB, the replace it with two sticks of 1 GB each. You could buy 2 sticks for under US$50.00.

You are already operating in single channel mode. While I agree that adding 2 1GB sticks along with 2 existing 256MB sticks is preferable (dual channel support), adding a single 1GB stick should cause no problems and you should see a performance increase.

Dual channel only increases memory performance a small amount (about 5 to 10%) not the significant amount many people think. There is almost no effect on gaming performance.

For best single channel performance, place largest module in first slot, second largest in second slot, and so on. First slot is usually the one closest to the CPU.

please show me a link to site selling it that cheap