Well looking at Microsofts page if you got ulitmate you have to upgrade to ultimate.The retail version of Win7 has both 32 bit and 64 bit in the box. Do you REALLy need win7 ultimate? It's pricey. Pro has most of the features you would need.
So if i format computer i can upgrade to 64-bit?Yup, I don't think you can upgrade to a windows 7 below what you have - so you'd be stuck with Ultimate. However, as Chet says, ultimate is very expensive indeed. Also you can't upgrade from 32bit to 64bit at all as far as I know. I think it would be worth doing a clean reinstall of Home or Pro 64bit. Last time I checked the difference in price between upgrade and full was very low indeed so it might not be worth going to the effort to upgrade.
Even if i want to upgrade from 32 bit to 64 bit, and buy a cheaper version than Ultimate.You don't have to format first either, you can do a clean install of win7 from within vista, it will back up your files for you as well.