I've got the 640Mb. Much better than the 320Mb. Worth the extra. Get it!
Don't bother with the GTX unless you have over 22" of monitor.
Sean
Its not just the size of the monitor but the resolution. If you have a 17inch monitor and wish to run high res and have what is needed to run FSX, get the GTX
If you are running 1280x1024 or anything below 1600x1200, and do not have the processor to keep up, then the GTX is not worth the purchase.
Lets not post things that suggest a GTX is not worth the purchase unless the stipulations are addressed. It is worth the purchase if the processor and resolution being run are correct for the card. Even the 640 needs right processor, bus and resolution to run correctly, which is 1600x1200 and above with a AMD 6000+ - E-Q6600 or above . If you do not meet those specs even the 8800 640 is a waste and not allowed to the resources it needs to run correctly.