A friend of mine bought an 8600GT from a competitor and an 8600GTS arrived at his doorstep.
He seemed somewhat impressed by the card since it costed him the best of £80, but to be honest I can't say an 8600GTS is worth it's £130 pricetag. Generally, you'll find that if you overclock your graphics card, an 8600GT can hit the same core limit as the GTS, despite being clocked so much lower. The only thing is that 8600GT's come with memory rated at lower speeds so you'd be a few hundred mhz short of GTS speeds.
On the whole, given that the 8600GT is hardly behind the GTS performance wise and can be clocked near GTS speeds, it's the better buy especially when you take into account that it's as much as £50 cheaper.
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With that in mind, used Radeon X1900XT's and 7900GT's can be had for around that much used, which can take down the 8600 series at stock speeds and pull well ahead when overclocked. You'd have to live with the lack of DX10 support but lets face it, there aren't many DX10 games around and won't be in large amounts for quite a few months.
My money would be on a used X1900XT/X1950Pro or 7900GT/GTO/GTX
Personally, I'm making the move to Core 2 Duo and have ordered a GeForce 7300GT as a stopgap solution for a few months. There's a chance that I won't get a DDR3 version, but oh well. Won't have to put up with it for too long
Mul