The 900 is one of Antec's gamer's case, so it should be able to fit a 9800GTX no problem. If it doesn't shame on NVIDIA/ATI for making such big cards and shame on Antec for not being able to accommodate a 9800GTX.

If you're planning to get the 900, my friend has one and it's only been a couple of months since he built that computer, but his fans are FILLED with dust. His 900 is a dust monster. You'd have to vacuum it every month or two to keep it dust free.
On a side note, I noticed that the Antec 300 and 900's dimensions are more or less the same. However the 300 is classified as a mini tower that can fit ATX motherboards boards. If the 300 is more or less the same as the 900 in terms of dimensions, why not just get the 300 instead. I'll probably get the 300 for my future computer. It's cheap, good build quality, what not to like about it?
Impressively spacious and unbeatably reliable, the Antec Three Hundred is an unbeatable enclosure. Despite its eminently portable frame, the Three Hundred has enough room for an NVIDIA 8800 series graphics card, as well as six hard drives and a standardATX motherboard. A cable management compartment keeps your system organized and the airflow unobstructed. A 120mm rear fan and 140mm top fan ensure that the case runs cool even when playing the most demanding games, while the sleek black finish gives it an extra stylish edge.