Mountain Biking - F/d R/d?

Im looking into buying a new F/D and R/D. I know little about derailleurs. To my knowledge there are 2 types of R/D long cage and short cage i believe they're called. And for F/D there are 3 types? i forget the names. How would i know which type of derailler I would want?

BIKE: TREK 4900 '02
FRONT DERAILLEUR: Shimano Alivio
REAR DERAILLEUR: Shimano Deore SGS

Unless they are broken, dont replace em. But if you want to know:

Front derailleurs are either top pull or bottom pull. Then one of those two are either top swing or bottom swing. The fd doesent get used as much as the rear so if you were to replace one, replace the rear one.

I actually just bought this today:

http://pricepoint.com/media/Shimano_XTRearDerailleur.gif

Deore XTR is the best you can buy but might be overkill for some ppl. I opted to go with the XT, at 34 bucks online its a steal. My lbs is trying to sell it for 80!

ya i went to my 3 lbs to shop around. lowest i found was $88. Im suspecting my rear MIGHT (big might) have somthing wrong with it. my friend is gonna check it out tomorrow. i was just wondering what the differences were.

ps. what online store was this?

i think jenson has the best prices on parts right now. i think they are in stock too

Go with Sram X.7

There have been good reviews of the X.7 and X.9 systems. There are even models of shifters that are compatible with shimano derailleurs - FINALLY.

Even so, I have had good luck with Shimano and don't plan on switching. I have found that Jensonusa almost always has ther best prices on equipment. Supergo and even Pricepoint seem expensive compared to other online sites most of the time. Performance has a larger selection than all of the sites listed, but usually isn't the cheapest uynless they happen to have a good sale on something.

Some other stores to check out would be Coloradocyclist, Nashbar, and Cambriabike. I've never been impressed with Nashbar and colorado cyclist is usually expensive, though they have some unique items.

-Moab

Cambria has an awesome selection, but their prices on some things (Like tires!) are freaking outrageous. They're selling the Giro Switchblade for like 180 bucks, and it goes for like 120 everywhere else online.

joe, if it isnt broken, a good investment is new tires. My specialized enduros were a month hold but I still picked up a pair of WTB Velociraptors. These babies are amazing. I am able to go faster, corner sharper and climb steeper with them.

i have xtr on both, but my bf had xtr on the rear and lx on the front...? the lx died first time he went over a tough trail... and altho specialized said xtr would fit his frame- it didn't so he is getting xt- which from all the research i did before buying my bike is extremely good value for your money....
but if they aren't broken....?

*futurerama professor voice* GOOD NEWS EVERYONE! lol it's not broken it's fine so i guess im ok. thanx

ya i know that jensonusa site rocks. unfortunartly i live in Ontario, Canada

If it ever breaks, I vouch for the XTR front derailleur. I know it's spendy, but just handling it makes you see the solid construction of the cage, as compared to others, even XT.