Mainstream NVIDIA GeForce 6600 GT and ATI RADEON X700 XT/PRO cards are located a grade lower on the performance scale.
The GeForce 6600 GT and the RADEON X700 XT match each other’s performance, even though the latter processes pixel shaders faster and has 6 vertex processors against 3 such processors of the former. In all probability ATI reduced severely the size of the texture caches of their new mainstream GPU, which negatively affected its performance with textures and the speed in some games.
Anyway, the ATI RADEON X700 XT feels better in the “eye candy” mode than the GeForce 6600 GT. We don’t give definite recommendations here as you should choose a mainstream PCI Express card basing on your own particular needs, like with the top-end devices. Both these cards have the same MSRP, which starts at $199, but the availability of the RADEON X700 XT deserves to be more widespread.
Fans of Doom III and of flight simulators will probably prefer the NVIDIA GeForce 6600 GT, but if you’re into Half-Life 2 and Counter-Strike: Source, you may want to choose the RADEON X700 XT (of course if you can find one in shops, as ATI is rumored to have abandoned the X700 XT project in favor of the more powerful RADEON X800).
The ATI RADEON X700 PRO differs from its XT mate in lower frequencies and a bigger memory amount (256MB). The price of the two being the same, the X700 PRO may be interesting for overclockers since some samples can speed up to the clock rates of the X700 XT.
Still, given relatively low chances to find ATI RADEON X700 XT graphics cards for sale, the majority of you will probably choose NVIDIA's GeForce 6600 GT, as it delivers excellent speed and is widely presented on the market.