Well, it entirely depends on your definition of smooth. If you are like me, you have a high framerate tolerance. I can find games playable at 20fps+, but others can't play games at less tha 40fps. But for less than or equal to $150 you'd be hardpressed to find something with the same amount of power.

A 6600GT is really pushing it on Oblivion, if you want the best graphics it has to offer. My 9800 Pro can sometimes sneak down to 4 or 5 fps in highly detailed areas. But in other enviroments it keeps a constant 30. Mind you, that is with all the graphics settings except antialiasing at there highest. The 6600GT should ordinarily do slightly better than my card. But if you seriously rack up the graphics, the bottleneck 128bit memory interface starts to show its true colors. After that, my card would slightly overtake yours due to its 256bit memory interface. I'd say that if you put the graphics on medium quality you shouldn't have too much trouble.

Another option is the GeForce 6800. I noticed that it has come down in price significantly but I don't have any experience with that card. If it is as fast as I think it is, for about $160 you could have much better performance. I suggest reading some reviews and benchmarks on the 6800. Keep in mind that I'm not talking about the 6800GT or XT, or Ultra, or whatever. Just the plain old vanilla 6800. If you like what you read in the reviews, I would definitely spend the extra money on the 6800.

If you have the time, compare the 6600GT and the 6800. Then if you have anymore questions just ask.

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If you have any questions you can email me at zack[ d o t ]schmid( a t )gmail[ d o t ]com.