Cyclocross - Thoughest terrain where you had driven your cyclo-cross bike

I'm deciding between a modified mountain bike (tweaked for better performance when running on paved roads), or a cyclo-cross bike. I need to know what are the limits that you can handle with a cyclo-cross bike when running offroads. Please give me an example of the thoughst, harshet, roughest terrain where you have ridden.

I'm not planning to ride super technical-aggressive-hardcore-downhill offroads but the dirt trails where I'm planning to ride can get pretty bumpy sometimes.

Most of the cyclocross bike models that I had check have only two chainrings in the front, how good are those bikes going uphills for persons like me, that dosn't have perfect physical condition. There are lots of hills where I live on paved roads and offroads.

Service on the local bike shops in my area that I have found so far, is very bad. I need to determine the right bike size by myself and I notice that cyclocross frames are a bit different than road frames (top tube lenght shorter, etc). On web sites like http://www.wrenchscience.com you input you body measurements and you got a recommendation on proper size for ATB and road bikes. Do you know of a web site like that but for cyclocross bikes?

I will appretiate your help!

Edil

You can get triple rings on cyclocross bikes. CX bikes like Kelly are designed for triples.

CX bikes can handle pretty much anything within reason. If you ride XC, a CX bike could handle alot of the same stuff. It wouldn't be as comfortable as a mountianbike though!

If you are mostly riding road, fireroad, smoother singletrack then CX is good. If you are doing more climbing, rock garden, drops, bumpy/rocky or sandy terrain and not too much on-road or fireroad than a MTB would be better.
THe rougher it is, the better a MTB is.

I agree with RacerX completely, could not have said it any better, words to live by :)

I use my Kona Jake the Snake a lot for river bottom singletrack riding which is fairly technical, but not super technical, so it works great. For these trails, a double works fine, but you can get triples if necessary.

I rode with these guys (http://www.strongframes.com/) in Bozeman recently and, well, I've never seen anyone ride a cross bike on the stuff we tackled - one of them was on a singlespeed.

My Poprad has a triple in the front, and the Lemond bikes are built with a longer top tube for a more relaxed stance. I don't have a long torso, but the bike still feels good to me. You can do like I did and get your LBS to order the frame and build it up to your specs.

I haven't done any hardcore off-roading, but the bike feels like I could. It is very solid. I have hit some bad pot-holes and such w/no problems. It's also fun to ride obliviously while the people around me on their Sevens and Cannondales yell out "gravel!".