Currently, I have Two 1GB Corsair DDR2 800 Ram and they are running in dual channel. I am thinking to upgrade to 4GB but then I don't know if all the 4 ram will run in dual channel.  What should I buy in order to run in dual channel(Faster, right) ?  The other 2GB needed to be same brand, frequency,latency as the corsair or I can buy different brand?

Ideally, all sticks should have same frequency, latency, and rated voltage. But if not, it'll still run, and in dual channel. The slower ram will just drag down the faster ones to the lower speed.

The other 2gb need to be in 2x1gb sticks, not a single 2gb stick, in order to run all in dual channel.

I have this at the moment.
 
http://www.directron.com/twin2x204864c4.html
 
This seems to be cheap at the moment.  
http://www.newegg.com/Product/Prod [...] 6820227139
 
Both have latency 4.  
 
Are they almost the same product? Will they run in dual channel? If you at my situation, which one will you pick?

You would need 2 sticks preferably of the same speed as what you have now as dagger said. The flexibility that you have depends largely on your chipset.

Desktop Boards - Single / Dual / Flex memory modes

They'll definately run in dual channel. Look similar enough, should work fine.

Thanks for the reply.  Really appreciate that.  
 
Oh ya, I got a last question. I am still using XP at the moment, is 4GB enough for Vista? I am planning to upgrade to Vista when I feel want to. Should I get more  than 4GB of ram?

The sticks only need to be the same in the same bank. The reason for getting all of them the same speed and timing is so that no sticks are under clocked or are set with looser timings. So I agree yes.

Vista 32 will only see ~3MB the same as XP 32. Four gig is more than enough for Vista 64 unless you are doing RAM intensive tasks.

Thanks

I should say Vista 32 will see all 4G as of SP1, I think. This is because of all the people wondering where the 4th gig went. Although it will see all 4G it will only be able to address/use ~3G.