Actually, the loss of 128 MB on the GeForce 8800 GTS 640 MB is inconsequential. The real differences are as follows:

• There are 32 fewer stream processors (92 down from 128)
• Core clock speed is 75 MHz slower (500 MHz down from 575 MHz)
• The shader clock speed is 150 MHz slower (1.20 GHz down from 1.35 GHz)
• The memory clock is 200 MHz slower (1.6 GHz down from 1.8 GHz)
• The memory interface is 64-bit narrower (320-bit down from 384-bit)

This means the GeForce 8800 GTS has a raw processing power about 35% slower than the GTX, and about a 35% slower memory transfer rate. When it comes down to it, 128 MB of memory accounts for very little in the context of >512MB.

Having said that, it's certainly more cost effective; you'll notice the price difference is 40~50%. You definitely get less bang for buck with the GTX. As for pricing, as I said, you'll have to price it up yourself. Depending on where you are, and if you decide to shop locally, prices range considerably.