SB86i and the 7900gtx

Hey guys

Wonder if anyone can help? i recently bought the SB86i and then bought the 7900gtx 512 thinking it would be cool to have a really good graphics. But as im a noob when it comes to comps, i found out that it is way to big to fit. So could anyone give me any suggestions eg. how i can make it fit or a shuutle which it will fit and that takes the same ram and pross so i can save money???

Sorry if i have posted wrong or nothing makes sence but this is my first post : )

Thanks for any help