From now on, more and more future games will start carrying the "Games for Windows" logo, ; windows = windows vista. No point in staying with XP. But the 4850 should definitely work with XP, if you have good overall hardware.
You'll be fine with XP, just as long as the game isn't listed as Direct X 10 only. "Games For Windows" doesn't mean Vista only. Read the back of the game box. Developers would be stupid to make a game DX10 only at this point, especially with all the backlash against Vista.