Assuming you mean 2004 iMac (available through May 2005), the only way to 'upgrade' the video card is to buy a new machine.
Assuming you mean PowerMac instead of iMac, there are a few options:
9600Pro for $220
9800Pro for $250
X800Pro for $330
The 9800 is probably the best value of the bunch, but I'd consider the X800 if you have the cash. The GPU (graphics processor) is more important than the amount of video RAM when it comes to performance.