I think the graphic card is fixed to the motherboard in such a way that DIY will be really hard.
Sending it in for repair would be much better and would give you some garanty.
First, i would go hear myself how much it is to replace the graphics card, than... if it cost more than that guy say, try to get some of the price off
Not sure if you would ever find a replacement graphic card for iMac CRT's online, anless someone sells his iMac off cheap for spare parts.
Others might have better info