The 8800 series were "refreshed" in Q4 2007, the first 8800GTS link you provided is the OLD G80 core 8800GTS. The 8800GT was released, which beat the old G80 8800GTS 320MB and 640MB versions, and the new 65 nanometer G92 core within the 8800GT (512MB) would carry on to the new G92 8800GTS 512MB, which features a dual-slot cooler as opposed to the 8800GT's single-slot cooler. It's a complicated naming scheme, probably deliberate by Nvidia to aid in selling old stocks of G80 cards through unknowledgeable people who think an 8800 is an 8800.
Your second link is a 256MB 8800GT, which performs slightly slower than the 512MB version, but it would do you good depending on what resolution your monitor is at (best value at 1280 x 1024 resolution monitors)
By far, the 8800GT should be considered above the old G80 8800GTS, don't let the price of the 8800GT fool you, it costs less and performs BETTER than the old series.
It's best to look at the memory amount, as you stated, the 512MB and 256MB G92 8800GT/GTS are newer than the older 320MB and 640MB G80 8800GTS.