1. Buy the Athlon. Get the best one you can afford. If you can spring for one of the new Athlon XP's, even better.
2. While 256 is enough right now, RAM is very cheap at the moment and you may need more in the future. I'd go with 512 MB if you can afford it. It's not necessary at the moment though.
3. The GeForce 2 64 Meg DDR is a great card, and the price is not too bad. However, take a look at the GeForce 3 Titanium 200. It will perform about the same as the GeForce 2 Ultra, but will have the updated feature set that may pay off later. If the price is close to the same, get the GeForce 3 Ti 200.
4. That's debatable. While 98SE is likely the fastest right now, it's not very secure or stable. I recommend Windows 2000 Pro, although Windows XP will be a good choice as well. With all new hardware, Windows XP would be my choice. XP will be the way to go in the next 6 months or so certainly, so you might as well get it now and avoid having to upgrade. Go with XP for a new machine, it will be the best choice in the long run, even if it's not the fastest right now.