lucky for you that todays midrange cards provide MORE power than that 8800 GTX did. Technology marches forward pretty quick, and as you can see cards like the GTS 450
compare very favorably to the old 8800 GTX, and can be picked up for about
$130 online, and some will even offer factory overclocking (such as this one) increasing the clock speeds significantly (from 783/1566/900 to
850/1700/1000). So as a direct replacement, it's going to have the same sort of performance as your 8800 GTX offered, many games will run smoother and support higher detail settings. Stalker Call of Pripyat for instance supports DX11, and DX11 runs and looks better than DX10 or DX9 modes in the x-ray engine
If you would like significantly more performance than your 8800 GTX ever offered, look towards the GTX 460 1gb, such as
this EVGA model for $240. And as you can see it really
outclasses the 8800 GTX.
On the ATI side at these levels you have the HD 5770 and HD 5850. The 5830 sits between the two in terms of price and performance, but if you want cheaper the 5770 does just that, if you want more power, the 5850 is where to look. Prices are similar to what you see from Nvidia, as are supported features such as DX11, use of GDDR5 memory, and so on.
I know you asked about used, but I thought it best to point out the new options as the 8800 GTX launched at over $500, but that performance can be had today for a hell of a lot less than that, and cards twice as fast (such as the 5850/GTX 460) can be had for half that price.
If you are looking used you could probably track down a GTS 250 for $50-$80 (new can be had for $80-$95 with rebates), or track down a GTX 275 which can be had for about $120-$150. Performance wise its not far off from the 460/5850 offers in many regards, but it is a more power hungry card, produces more heat, typically more noise, and it is rather long at 10.5in so it may not even fit in some smaller cases. From ATI you could probably track down an HD 4850/HD 4870/HD 4890 for similar prices. But I would advise looking towards some of the newer cards as they are simply better in terms of efficiency, supported features, performance, and virtually every which way.
Hope that helps.