Um, that X800 is NOT the best bang for the buck. The average 6600GT is under $200, while that X800 gets near $300. You could ALMOST get an AGP 6800GT for that price. The X800XL is a high end GPU, where the 6600GT is mainstream and value based.
You won't get a better buy than the 6600GT, that X800 is a little more than $100 MORE EXPENSIVE. Stick with the 6600GT, it IS the better buy.
As for the components fitting into your case, yes, they WILL fit.