DirectX is software from Microsoft for running games on Windows.
basically, it works closely with the drivers, and is the bridge between the hardware (video card, sound card, CPU) and the game you are running.
wait about a month, and save more money.
once ATI's next card comes out, go and look at the benchmarks and find out which card comes out on top.
if the ATI card is better, get one of them.
if the 8800 is still better, get one of them. once the ATI cards are out, they should be significantly cheaper.
I have a feeling the ATI card might be better, but don't quote me on that.