Bicycle Mechanics - Derailleur rubbing on the chain ring when I pedal hard

When pedaling lightly no problem. But when I start to put pressure by either going up hill or a sprint they rub. I was thinking it may be the cranks flexing under the stress.
The bike is a 2000 RockHopper pro. Alum frame and cranks

Yes..

How would I go about looking for a new set of cranks? What should I look for and what questions should I ask?

You didn't say what crank you have now! Problem is that any good step up crank for a MTB is expensive like the Shimano XTR at $450; you could try the Shimano XT unit and see if that works. Another problem is that if it's frame flex and you spend money on a new stiffer crank-your still going to have the problem. You should take the bike into your LBS and have it checked to make sure frame flex is not the issue.

But I don't know that much about MTB's so I'm just guessing.

Did you try adjusting the derailleur... or is it an obscene amount of rubbing?

Actually I think it's the bottom bracket that's flexing, not the cranks ... or maybe a combination thereof.

I found the new Shimano XT M760 cranks/bb combo set to be extremely stiff ... if there's any flex at all, it's the frame (it's an old Trek 7000 mtb frame). I also weigh about 135 lbs so I'm not that hard on equipment.

Some flex is normal. If it's really bugging you, I'd go for the new set.

I wouldn't use the XTR cranks if you have concerns about longetivity ... the chainrings are all aluminum (for weight savings), so they wear faster.

It's your chain length or missaligned deralier. Has nothing to do with flex.