8800 GTS 320 or GTS 640 are older, overpriced, "slow" cards.

The correct new GTS is the 8800 GTS 512 using the G92 GPU.

This is similar to the 8800GT, but with some extra processing power
unlocked, and these cards have higher stock clocks on GPU, Memory,
and Shaders. They perform close to the most expensive older Ultra
cards except in games where the Ultra's extra memory bandwidth
really helps.

Older and overpriced? Not when they were introduced. Compared to the new
8800GT and GTS? Yes, but that was then and this is now. That's the usual
playout of any cutting edge technology. The stores still selling these are
selling stock that they too had to pay high wholesale price for and aren't
willing to take a huge loss on their stock.
As for speed and perfromance, any 8800 series card is within close spitting
distance of the others. There aren't any crappy ones performance wise. And
with any newer hi-perf card, the real differences are magnified when
resolutions and eye candy is above the 1280x1024 level. A G92 8800GTS is not
the equal of an 8800 Ultra simply because default synthetic benches and game
framrates are within 2-5% at 1280x1024 and under. There's a couple of newer
reviews comparing the G92 8800GT and 8800GTS with the GTX and Ultra, and
where the GTX and Ultra shine is at the high end of resolution and the
newest most demanding games.