At this point in time, I'd say the 6600GT is your best bet. The fact is, the 9800Pro is still a good card, but it's definitely already showing its age, particularly on newer games, i.e. Doom 3, FarCry, and HL: Source. These engines are the engines of the next few years, so I'd be worried if buying one today. Also, if you do decide you need to upgrade in the future, your options on AGP will be pretty limited. On the other hand, the 6600GT, and even the X700XT to an extent, give you something on par with 9800XT performance, for 200 bucks. Also, if you need to upgrade, you can easily find a PCI-express card, or throw in another 6600GT for cheap, if you have an SLI board. nForce4 looks great, from its hardware firewall (low CPU usage) to its SATA-II support. And nVidia has stated that the nForce4 chipset has been tweaked to run GeForce 6 series GPUs optimally. I doubt it's going to be a huge difference, but I expect it'll be in the neighborhood of 1-2%, which is nonetheless a good thing.
One last thing - I'd recommend looking for a lower end CPU before cutting money from your video card. If you play games, any money you spend upgrading your video card will yield much better performance than a faster CPU. For example, the difference between the 3200+ and 3500+ is 80 bucks, while the performance gains are roughly 8%. Look at the percent difference between a 9800 Pro and a 6800 - also an $80-100 difference, and you yield speed gains of around 30-50%. Also, the 3200 and 3000 are 90nm parts, and overclock really well - 2.6GHz is not unreasonable for either model, on only high end air.