Mountain Biking - shimano chainring differences

I ride a 98 Rockhopper that I recently bent the outer chainring on, the others are rideable but I also want to replace the middle ring because it looks slightly tacoed and is missing a tooth or two.

Checking Bikepedia I see that my bike has no particular groupset; the chainrings, cranks, brakes and deraileurs are dubbed as "mountain mix" components so I'm guessing that I can't just buy new replacement chainrings. I want this new set to last another decade so I'm looking into buying Shimano deore rings in 22/32/42 (my current configuration).

The deore LX 42 tooth looks like my choice for the outer ring since its way cheaper than any other model, but I see that a 32 tooth XT chainring is not really expensive; will there be any benefit in having a LX big ring and XT middle ring? Will I notice the difference from mixing types, rather than just buying LX 32/42 rings? In fact, the XT granny ring is only $18, would having XT inner and middle rings + LX outer ring make me lightning fast (or just give me an advantage)?

Also, if I was to change the gear ratios, how would I decide which rings to make bigger or smaller?

It may be hard to ensure that new chainrings are compatible with your existing cranks. The most economical thing to do might be to replace the entire crankset. If you have a square-taper bottom-bracket spindle, just beware that there are two incompatible standards:
http://sheldonbrown.com/bbsize.html#square

What gear-ratio changes are you thinking of? Changing the sizes of the chainrings will have consequences for the chain-length and the derailleurs.

It may be hard to ensure that new chainrings are compatible with your existing cranks. The most economical thing to do might be to replace the entire crankset. If you have a square-taper bottom-bracket spindle, just beware that there are two incompatible standards:
http://sheldonbrown.com/bbsize.html#square

What gear-ratio changes are you thinking of? Changing the sizes of the chainrings will have consequences for the chain-length and the derailleurs.

Actually there are quite a few incompatable standards when it come to chainrings these days.

There are chainrings with different spider BCD (Bolt Circle Diameter) diameters - 110mm vs. 94mm - for the 2 outer chainrings.
There are chainrings with 4 bolt spiders and 5 bolt spiders.
There are some granny gears with 58mm BCD and some with 64mm and even 75mm BC.

And these are just current Shimano specifications. Add in Campagnolo and some of the other odd and obsolete stuff out there and you have real problems.

Best bet if you aren't sure what to get is to take your old chainrings to the LBS and ask for quality replacements. At the very least get them to tell you what you have so you can order them on the internet (although this isn't really a good idea if you don't know what is good and what is mediocre or even poor quality).

If it helps, my bicycle has a Specialized "Strongarm Expert" crankset. I'm going to the my lbs now to work on another bike and I'll ask the owner these same questions; while I support my local bike shop, I'd prefer to buy the new parts online.