the 6600GT will be the best bang for the buck meaning best performance/price ratio, I'd assume especially if you got eVGA brand you could OC it and get better results, but yeah it does have fewer pipelines, but it's still going to be one helluva card and is far from 'old technology' as it blows away any old technology completely, having that GDDR3 memory helps and the fact that the 6600GT I checked had 500core/1000memory speed which is pretty dern good.
All the reviews I read on it said it was awesome and performed better than they had expected especially in doom, reaching 1280x1024 resolution, high settings, 2xaa and af if not more ( I really can't remember) and apparently held out like a champ so I personally think I may look into it when it comes available in AGP from eVGA