Hi Marek, good to see you out the other day (am way below your league normally so never see you on the road CRs
). Glad you enjoyed it.
Second Marco's view on the various makes. Kona go for slightly stronger bikes and you see a lot of them around Leith, Afan in Wales and Gorrick where there are mtb races. Trek and Specialized have big pro race presence so there is good trickle-down of technology. Also the sale option is good ie picking up 2008 models at bargain prices from eg Evans. However don't expect much GB-style advice from them unless you plan hopping around a shopping centre on a bmx. Sizing is less precise than on road bikes as you're constantly moving about so try a few in the 20"-22" range to see what's comfortable.
Re kit, Shimano XT is about equivalent to Ultegra or Campag Centaur/Chorus, LX Veloce/Centaur/105. It is extremely hard wearing - my LX front mech and XT rear are 9 years old and the only original parts on my mtb. XTR is DuraAce/Record equivalent and quite expensive. Re forks, Marzocchi are excellent and they do good air/coil ones which last forever and are infinitely serviceable. My Marathon S forks are 4 years old and serviced once in that time. They also have good resale value should you want to upgrade. I sold the original Marzocchi Atom Bomb forks on ebay after 4 years use for £75! The firm has been in business since the forties so know what they're doing. Hydraulic discs rarely need maintenance and are great in the wet. Generally you get 180mm rotors up front and 160 in back. I run smaller at 160/140 but then I'm only 5'8 and 65kg.
AS for dual sus vs front only, you get a better bike for your money with front only suspension. The bikes tend to be lighter and quicker and there's less to go wrong.
Finally pedals - double sided shimano spds, eg 520s are fine. Most people run shimano pedals so if ever you want to swap rides with your mates (always best not to ride your own bike across a frozen pond!) you can do so easily. Not compatible with road shoes but mtb shoes have grippy soles for the odd time when you need to walk. Specialized BG comps are very good shoes, but again get what fits and feels right.
Hope that helps a bit, and welcome to the dark side.