Choice of bike is of course very important...

But...

The other thing if you are having back probs is asking yourself how strong your tummy muscles are...

Remember, between your ribs and your hips the only bones holding you upright are in your spine, so strong tummy muscles go a long way to reducing back pain - particularly lower back pain.

The nature of cycling will work-out your tummy muscles, and if they are not strong, your back may suffer

So, in conjunction with trying to find a "good fit" bike, find an exercise that will support and strengthen your lower back and tummy... I prefer tummy crunches (sit ups hurt me). In the Jan-Feb 2006 edition of Bicycling Australia is an article on strengthening your torso for cycling, and that includes some standing/leaning exercises and some exercises using a pilates ball.

Good luck with bike choices and with getting your body used to and comforatable with cycling.