Long Distance Competition/Ultracycling, Randonneuring and Endurance Cycling - newer small bike or older big bike

I'm about to start training for a metric century that's happening in a couple of months. I've been doing some short rides to get back into it and have come to the conclusion my current bike is too small. The seat is almost at the top of it's post and I always feel like the handlebars should be further away.

I've got 2 bikes, a giant boldershock from about 2000 and a ??? donated to me last year. I have no idea what the other bike is so I'll call it the green one. The bouldershock has been a good bike to me and suits me well off road, but on a 100km ride I think I should go with a slightly larger frame of the green one for better comfort.

The bouldershock from floor to half way along top tube is 30", it's 32" on the green bike which doesn't leave much space for the fellas when I stand over it. The bouldershock has a top tube length of 21" and is angled, the green one is 24" and horizontal. The bouldershock has front suspension, the green one doesn't. The bouldershock has 21 gears, the green one has 15.

So my question really is, does it make sense to go with a bigger frame on longer journeys? (I'm pretty sure I read on this forum that it is)
The second question is how do I find out what bike it is and whether the components are any good? How do I know if it's worth replacing the naff components on the green bike or replacing the whole thing with a new bike. (I don't want to strip down the bouldershock as I'll use it for offroad.)

Here are some pictures of the green one incase anyone can tell me right off the bat that it's a piece of junk:
http://www.kentestman.com/images/greenbike001.jpg
http://www.kentestman.com/images/greenbike002.jpg
http://www.kentestman.com/images/greenbike004.jpg

Basic bottom-of-the-line bike but people have gone further on worse bikes...

For this sort of distance am I right to assume that a good fitting bottom-of-the-line bike would be better than a too small Giant boulder shock? I have no idea if the Giant boulder shock is considered a good bike or not, these reviews (http://www.mtbr.com/reviews/Bike_hardtail/product_19724.shtml)suggest it's ok with a rating of 3.8/5.

Something that is comfortable and reliable is the way to do longer rides. The Giant scores 50% apparently. If your green bike is both of those things, you've got your answer. I'd suggest fitting higher pressure slick tyres though.

I'd suggest fitting higher pressure slick tyres though.
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