Your Screen Resolution.

1600x900 on my main screen.

Triple monitor setup with 3 17" monitors running at 1024x1280 each in eyefinity.

1620x900 im pretty confused tho, when i play games, i see Reresh Rate. It was set to 60 but i see higher like 75. if i make it 75 what happens?

Nothing, you own a TFT monitor. Refresh rate has almost no effect on those, as they show a still picture between re-draws.
CRT monitors needed to produce higher refresh rates - preferably at least a 100 Hz - as their picture fades between re-draws, causing a "flicker" if the redraws didn't happen soon enough.

1360x768

Got myself a 1440x900, does pretty good for the most part, just has some scrolling line problems when it gets too warm. :P

1440 x 900. I think it could go more but I haven't seen any games that could exceed that.

Excuse me, but what the heck are you talking about? I could run Mechwarrior 2 on 1600x1200, which is a 16 year old game. It's just that raising resolution for the heck of it is not worth it, since you won't notice the extra DPI (dot per inch) since the size of your monitor is fixed.

If you say, 1440x900 is the "sweet spot" for your monitor then it's a sensible answer... however this has nothing to do with what games can do.

1366x768 and a 1920x1200.

1366x768 It's a laptop, so...

I use an 19" with 1280x1024 resolution.

21.5" at 1920x1080