I like the ATI cards personaly (I have an 8500 I've been using since they came out, been great for me). However, if you're talking an AGP 4x system, the CPU will be a bit of a limiting factor, so I'd probably go with whichever you can get cheeper (the 9800 ATI or the 6600 NVidia). Also, make sure you have a good power supply. Compair the power outputs (5+, 5-, 12, and 3 I think), not just the net wattage.
Better power supplies will often have higher outputs, and are therefore more efficent, which results in cooler running, better power consumption, and they generaly last longer. I wouldn't get anything less than a 400 watt for a new system (or a good 350+), and even an older system needs a good power supply.
I can't stress this enough, and while it's gotten better it's still an issue, a cheep or inadiquate power supply will cause stability problems in your computer, as well as potentialy damaging situations.
A good power supply will last. I've only had one of the PSU's in any of the machines I built myself since '97 go bad, and that was the first one I bought, and it was the lowest end one, and had other contributing factors beyond simply age (New owner smoked, and did not clean well).