Disregard the following if you want to keep your current motherboard/cpu
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There's a
12 pixel pipe X800 at newegg for $105 after $30 rebate.
There's also
this sapphire x800gto for $23 more than the msi x800 after the rebate but it has faster memory
Both of these are only 128mb but they have more pixel pipes/vertex shaders and run with 256bit bandwidth so should outperform a 6600GT the majority of the time.
The prices are the same not counting the rebate so, aside from any brand loyalty you may have, you'll be choosing either a faster card or one that supports some prettier graphics in one or two games (currently).
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If you're keeping the motherboard/cpu, the 6600GT is about the only reasonable choice at the $100-$130 price point. The only thing ATI has to offer in that range is an X700 and the 6600GT is superior.
If you find some extra cash, there's a sweet $195 256mb X800GTO in AGP flavour.