In the SPECS
just look at the amount of pixels that compose your screen which you can figure out by multiplying the horizontal count by the vertical count..
1280 x 1024 = 1,310,720
My 22" Dells are 1680 x 1050 = 1,764,000
and I can max out Call of Duty 2 all the way with a GTS 320MB, everything all the way up... and I still get decent (great!!!) frame-rates. (with my current CPU: an E6300 at 3.15GHz)
The GPU from the 8800GTS's are all the same... the 640MBs and the 320MB really only differ in the amount of memory they put on the card which is available for the GPU to use... think of it this way... the RAM is random access memory which basically is used to hold info momentarily... when you do a math problem you have to figure out things in steps, and you hold the results in your head while you process other parts of the problem, then you use the first result and the second one and you put those together... when you render a picture that is bigger (higher pixel count) you are rendering more pixels... so there are a ton more numbers that need to be momentarily held... and while the processor is fast enough to make all the calculations, there isn't enough memory to hold the info, so the GPU can only go as fast as the memory can go... more memory then allows for the GPU to operate at it's max speed, and it is fast enough create the 3D environment and render a 1920 x 1200 = 2,304,000
pixel picture at a rate of about 30fps in your average high end game...
I think that the 640MB are going to be a good deal once they drop an other 50 bucks... but if you will be asking the Card to render 1,310,720 pixels per frame... then my personal guess is that you'll be good (as in EXCELLENT) for at least two years with the 320MB version...
BTW, 700:1 is a great contrast ratio! Seems like a killer 19"