Currently i have ONE 9800 GTX but i'm planning to get another one (if i really need it) later on. But i've been wondering about how much better is a 24"(1900x1200) compared to a 22" (1600x1000)?
Is it really... THAT much better than I should pay $100-200 more?
It'll be useful if someone came be some statistic information about the difference in performance. (with SLI)

I was originally going to go for a 24-inch but I ran across the Gateway FHD2400 and fell in love with it. It is definitely an awesome monitor and the 1920x1200 resolution IS worth it in my opinion. The problem is getting the graphics horsepower. I believe two 9800GTX's in SLI is great for it but I would hold off. Two 9800 GTX's would be great for that kind of resolution but given the current graphics card market, I'd be aiming for the 4870 in CF but I'm assuming that you have an SLI motherboard and therefore  out of the question. Seeing how you are kind of locked into the NVIDIA camp,two 9800 GTX's or 3 would give you ideal performance, especially with the price drops, but I would also recommend waiting to get a new one for the next series of cards. The new GTX 200 series cards really aren't price justified IMO and are just an option if you are building a new rig or have all kinds of money to throw away. I currently am in your situation with one 9800GTX and the 1920x1200 display, and the 9800GTX takes care of business better than I expected, I am able to play CoD4 at max settings with only a few instances of noticeable lag (high brush areas). Because of this I don't really want to shell out the extra 200 for a 256-bit card and at the same time I don't want to shell out 400-600 for the 512-bit cards that perform roughly the same as 2x 9800GTX's so therefore I wait for the next NVIDIA series, which as of yet I have heard nothing about.

I got a 26" monitor which has the same resolution as a 24" (1920 x 1200). I mostly got it so that I can have more windows open 'cause of the larger real estate.
 
Higher resolution basically means you have a wider viewing range. It doesn't necessary mean the graphics will look any better.

 
 
I understand that, but I heard from other people that a 24" benefits more from SLi, compared to that of 22"?
and i'm just wondering how much more does it benefit?

Its a nominal difference, you can make gpu's a deciding factor at 22" by jacking up quality settings in the drivers.
 
Anyways, I own a 22" display and i'm perfectly happy with it - but if you can get a good deal on a 24" display, there is no reason not to.

Hmm... Running 2 9800gtx on even a 24inch monitor is kind of absurd lol even 1 8800ultra or gtx can run run that easily, for ur setup of 9800gtx I would get a much bigger monitor then a 24inch lol. I am running 9800gx2 in sli right now and every weekend I bring my PC downstiars to my 48inch 1080p lcd tv. Thats what your cards are designed for, and your not even making them break a sweat with that monitor. Yes a 9800gtx is less power then gx2 but it is still a monster card!

Two 9800GTXs would probably get somewhere near or above the performance of a 9800GX2, given that that's basically two 8800GTs in a single package and that the 9800GTXs are 8800GTs with higher clocks and 12 more SPs each.  And with that kind of horsepower, I think 1920x1200 is just the right resolution for that setup given today's games.  Just make sure you crank up the settings and get the most out of what you got.

Oops, accidently quoted myself.