Tigerdirect has plenty of 2G memory modules. Go have another look there. I saw a 667Mhz for $110 and a 533Mhz for $130 in just a very quick look.
Dual channel means they are optimal for a motherboard that has a dual channel architecture. Basically it means faster transfers, but honestly it can be done with any memory you put in there, it just works best if the two chips are identical. So they say "dual channel" and really mean "two identical chips of RAM".
I went to crucial.com and ran their scan. It told me I had only 2G of RAM, when I am sure I have three. The machine came with one, and I bought two at best buy that were 1G each. Not sure what's the deal there.
When the computer first starts up it should say how much RAM you have that it recognizes. Also you can right click on My Computer and you'll see it there or in the System Information program that can be found in the Start Menu, Programs, Accessories, System Tools. If it says 2G in those places then the motherboard isn't recognizing all of the memory. Most system wouldn't even boot in that case but some will and just ignore the extra. If they run at different speeds and the PC doesn't like that or if the first 1G was one chip instead of two 512's sometimes it hates that too.
There are some free programs that can give you details like RAM speed, but to me it's easier to either check the BIOS on startup where it can usually be seen or just pull out the chips and you can read it on them in most cases. It may be listed as sometime like PC4200 but if you look online you can quickly figure out what speed that refers to.
Your motherboard will only be capable of so much memory. 4G is a typical limit for a lot of boards. From what I recall, getting more than 4G is kinda pointless anyway if you aren't running a server. You can get faster RAM if you're running slow speed stuff and that can make a difference. Though I would doubt your motherboard takes the new DDR3 stuff, it might be capable of 800Mhz DDR2 or even 1066Mhz.