If you don't mind me asking, why do you want to replace like-for-like? The 8800GTX is a very old card (2006 from the top of my head). There are far superior cards available today that you should be able to replace it with.
Having re-read your post, I see I misunderstood what you said. If you're looking for a replacement then anything from a GTX 650 from the Nvidia range or a Radeon HD 7750 from the AMD range will be an improvement. What is your budget?