Yes, the 5900 beats the 9800 only on 1600x1200 resolution. The ATI card is nearly 800 3dmark2003 marks higher than the 5900. The only reason the FX 5900 beats the ATI card at that resolution is because of the 256MB memory, but you can get an ATI card that has that mem. if you want it.
Here are the things you need to look at:
1. Size. The ATI card is tiny compared to the FX card. Think about how much airflow you want to lose with this...
2. Heat. The ATI card has much lower speeds than the FX card. 380 compared to 450. That's the only reason Nvidia can keep up with ATI. Now I own a 9800Pro myself, but I have modded it with a Thermaltake Giant II heat sink allowing me to almost meet Nvidia's clock speeds. Now I score over 6000 3dMark2003, over 1000 points higher than the nvidia card.
3. Drivers. Nvidia has hands down better drivers, at least that was true until the release of the Catalyst 3.6's. I used to be an owner of a GF4TI4600 and I'll never go back to Nvidia becuase I got this card and am VERY happy with. If you know how to tweak it just right, the overall look of the picture quality is much better than Nvidia's.
I dont' want my opinion to be biased, but I have used both companies, and I'm hands down much happier with my ATI card then I ever was with my GF4. As a matter of fact, I had to send my GF4 back to the company because it was messing up and was proven to be bad. When I got the next card in, I still wasn't happy with it, and it was PNY. I couldn't use AA and AF at high resolutions. So I sold the card and now I can do all that I wanted and more.