Google "fork measurement", something like that, you'll get lots of into. On
most rigid mtbs as long as the steerer is the same/right length and it has close to the same blade profile(straight or radiuse) you should be ok, with little if any noticeable ride difference. If you have a high end and/or custom frame best to be a little more particular in the selection.
And you must be familiar with
http://sheldonbrown.com/gloss_ra-e.html#rake lots of info on forks,
Rake: The "rake" or "offset" of a fork is the distance between the wheel axle and the extension of the steering axis. This may be accomplished by bending the fork blades, or by attaching the fork ends to the front of the blades, or by tilting the blades where they attach to the crown.Rake is one of the three factors that affect the trail of the bicycle, which has a considerable influence on the handling qualities.
A fork which is curved forward at the bottom also adds a small degree of suspension compared with one that is not, by acting as a leaf spring.
Brian