Dukey: That's american dollars though. Likely a good deal for AGP users, as you say, although on the PCIe side I'd probably say the X800XL.
After the 9800 was the X series. The 9800pro and X700 are basically the same card performance wise, but with the AGP/PCIe difference. X300 and X600 are low end, while the X800 XT/XL variety is the high end.
Nvidia's line is the 6x00 series - the 6600GT (note the GT) being better than the 9800pro but competing in the same market segment, while the 6800GT competes against the X800 line (and fails/wins depending on where you look and what card you compare).
Nvidia's performance advantage right now is the GeForce7800GTX - it's their new generation card (so ATi doesn't have a competitor to it out yet), and it's basically 2x the speed of the 6800GT for 2x the money. If you want the best there is nothing better right now, period.
In general I think both companies are well matched - the X300 has some advantages over Nvidia's competition there, the 6600GT has a slight but definite improvement over the 9800/X700, and conversely the X800/X850 line has some improvement over the 6800. I think it's close enough lately - and I'm speaking as a guy with an X800XL at home and a 6600GT at work - that you should honestly go for the deal and not stick to a specific branding.
For tiebreakers, though, I would recommend aligning the video card and motherboard chipset when possible; ATi bridges with ATi cards are a lot easier to deal with than mixing ATi and Nvidia, and the same goes the other way.