I got a 9800 np, only issue I've had was in Need for Speed: Underground. Messed up textures on screen, just needed to increase the mipmap detail level.
I installed the Cat 3.6 initially, then using the ATi uninstaller, upgraded to the Cat 3.10's, no problems whatsoever.
My nVidia card (GF4 Ti4400), installed 29.20 drivers (one's I had on CD at the time), uninstalled them, got some updated drivers (43.something IIRC), installed, and they were a bit messed up for some reason, the control panel type thing won't work so I can't change display properties, has happened before, just needs the drivers reinstalling. nVidia is so good
I've had no real problems with either ATi or nVidia, I would go witha 5900XT though (they're something like $200) as they are the best price/performance card, unless you can snag a 9800 non-pro, which is a rare card now, but also great value for money. The higher end cards (which just have greater clock speed and stupid price tag) and not that good a deal really. Neither card is a good option. The 5900XT and 9800np are the way to go.
A 5900nu is just a 5900Ultra with lower core/memory clocks, and with a bit of overclocking, the 5900nu will be equal to the 5900Ultra.