Need a little help buying extra ram

Buy 2 identical dimms to run dual channel. You have dual channel now, why make your system slower? So (2) x 1GB would be the choice given your stated options. This will give a total of 4GB in your system, which should be more than plenty.

hi newf, tx for answering.
The problem is that my supplier doesnt sell dual channel at this moment. I know that sounds silly, but i dont have a store here at every corner.

What you're telling me is that the only way to run dual channel on my current system is by buying 2 extra 1GB Dual channel dimms
For instance:
KHX6400D2K2/2G ; 2GB kit DDR2 800MHz Non-ECC CL5; (kit of 2 - 1GB)
Thats actually bad news, that means that if i don't want to throw away my current ram i can never have more then 4GB in my system, since i only got 4 slots.

As i understand it one can do the following (with 6400 dimms)

1)Running dual channel, at 400mhz, need less Voltage, but higher latency
2)Running Single channel, at 800mhz, needs more voltage, lower latency
3)For dual channel you always need dimms of the same size, and they need to come in pairs, so only 2*1GB or 2*2GB or 4*1GB or 4*2GB, not (2*1GB)+(2*2GB)
4)IF 1,2,3 are correct the only advantage of dual channel is less power consumption, so i guess there is something i dont understand?

I dont' know where you are getting 1 and 2 from, these assumptions are offbase. 800mhz DDR runs at 400mhz bus speed, period, whether it is 1, 2 ,3, 4, 5, no matter how many sticks there is. Bus speed has nothing to do with dual channel or single channel. Perhaps your are getting confused with bus bandwidth, (64 bit/128 bit)which is what DDR has to do with.
In other words, single channel gives you a 64 bit wide bus, dual gives you 128 bit. But in real life, there is not a big speed difference in the 2.
Memory must be installed in indentical pairs to run dual channel.
The 4 slots on your board are actually 2 "banks" of 2 slots. You must have a matched pair of memory sticks inserted into a bank to run dual channel.
You can run dual channel with 2 or 4 sticks or memory on your board. You cannot run dual channel with 1 or 3 sticks.
Dual channel or single channel has no bearing on latencies or voltage, but occasionally, 4 sticks does require a slight voltage increase over stock simply because there are 4 sticks to run instead of just 2, just more overall system overhead is being used.

Unless you are running a 64 bit OS, you cannot ever use more than 4 gig of memory anyway. If you are running a 64 bit OS, you can then install and use more than 4 gig, but 4 gig of memory is way plenty for all but the most hardcore users.

If you want to add 2 more sticks of memory, you are best advised to buy more of what you already have. Some memory simply does not work well with other memory.
If you cannot buy the exact same memory, you must be concerned about and at least buy memory that runs the same voltage, you can have a whole host of problems by installing memory of different voltages.
Lastly, if you must, just yank out the 2 sticks you have, and buy a set of 2, 2gig sticks and put them in. I know it seems like a waste, but it is a way better situation than mixing 4 sticks of different brands and timings, AND VOLTAGE.

Thanks alot for your answer jitpublisher,
I think i will just do as you say, removing my old RAM and i'll buy 2 sticks of 2GB dual channel. A good thing is my bro needs an upgrade to so then he can just use my old Ram, tx again for the info
jorgev