I know this same question has been asked a lot lately so apologies first of all !
My take is slightly different though as I'm currently running a GeForce 5800 Ultra (and before anyone jokes about the Hoover attached, I replaced with a Zalmann Copper Cooler).
What could I upgrade this to without breaking the bank ? Is there anything in the £100 range that would give me any improvement ? I've started to get onscreen glitches and think the old dear may be on her way out after a good few years in action.