right now i'm building my itx pc, main problem is not the length of the card but how much power can i provide to it [...] i'm not a "gamer" but i do play from time to time, right now i have a 4850 with my current rig and it's ok...Will your ITX system have space for a dual-slot card?
If you want your new ITX machine to be as good or better then your current rig, you will have to be careful about size and power usage. Your AMD HD 4850 has 800 stream processors and 16 ROPs at 625Mhz -- current low-end GPUs will not be as fast as this powerful older card.
Small step down:
HD 6670, 480 processors, 8 ROPs, 800 Mhz, $95
Small step up:
HD 5750/6750, 800 processors, 16 ROPs, 700 Mhz, $110
Large step up:
HD 6850, 960 processors, 32 ROPs, 775 Mhz, $160