The difference between those two cards is that one (O3G) is factory Overclocked, thus has slightly better framerates out of the box. The other (3G) is not overclocked, but can be manually overclocked as well. You can take the lowest priced out of the two, and simply overclock it yourself later on if you wish.
They both should fit your case with ease. You can always measure your old card? I believe it was the exact same length, around 24cm (6.6inches).