XFX is just a brand name. You can't actually buy an "ATI" card, they're all made by brands like XFX, HIS, Sapphire, PNY, MSI etc... doesn't really matter which you buy because they're all the same card. Biggest difference is usually the cooling. Same goes for Nvidia cards.
9800GT is fine, it's basically the same as a GT240 just older. I'd take the 240 over a 9800.