There should be no reason whatsoever that the two shouldn't work together. Just because the nForce2 chipset is produced by nVidia all of the sudden everyone and their dog for some reason thinks an ATI product automatically won't work well with it. It's just a chipset meant to connect all the different parts you plug into the motherboard. Companies like Abit and Asus and MSI then make the motherboards with those chipsets. It would be a huge mistake for nVidia to somehow come up with a way that only their video chipsets could work with their motherboard chipsets because then anyone without nVidia video cards would be forced to buy an alternative chipset solution (IE the VIA's KT600). While maybe not as fast as the nForce its certainly respectable and will easily be enough to let users live without the nForce2.
The ONLY reason you would might want to get an nVidia video card over an ATI video card if you're basing decision off of platform type would be if and only if nVidia boards for some reason perform better with AMD CPUs and ATI better with P4s, but because CPUs pretty much do their own thing and GPUs do theirs (wasn't so back in the day of video acceleration where the CPU was still heavily used for graphics processing) and all the chipsets do is traffic the information between all the different peices of hardware, really there is no reason to believe that a 9800 shouldn't work very well with an nForce2 other than that ATI and nVidia are major enemies in the performance video market.
And if you're not so sure just do a little research on your own, you'll notice some pretty damn high 3Dmark scores based off of AMD + nForce2 + ATI setups and countless customer feed back from nForce2 users with 9700/9800s (just check customer reviews for nForce2 powered motherboards @ newegg.com)
I can tell you right now that my AMD CPU and VIA chipset are working just dandy with my Intel network card...
