The X700 Pro is a bit slower than the 6600GT, but it's available for about $35 less than the 6600GT ($143 vs. $178 @ 128Mb PCI-E). The X700 Pro is also available in a 256Mb version.
The X800XL at $269 (ATI Store after 10% discount) is significantly faster than the 6600GT and is normally considered a match for the 6800GT ($329) in almost all benchmarks except Doom 3.
The X800 card should be a bit faster than the 6600GT, but their still stuck around $249 street price instead of the $199 they were expected to sell for. The X800 was supposed to be available in a 128Mb version, but so far 256Mb versions seem to be the only ones shipping, so that might help explain the pricing discrepancy.
Pricing seems to be pretty linear on the performance scale, though. If you simply buy the best card you can afford from either chipset manufacturer, you should do pretty well. The only exception to this rule that comes to mind is the X800 Pro, which is a turd compared to most comparably priced competitors ($350 to $399).
Edit: Just saw your comment about having a $140 budget. That would put the X600 Pro, the X600XT, the GF6600, and the X700 Pro at the forefront of your shopping list. If you can't step up to a 6600GT ($169 + s&h), the X700 Pro is an excellent choice at $145.50 with free delivery (Newegg pricing quoted).