Hmm, some pretty strange advice in here :eek2:
Anyway, the Trek you looking at is a pretty decent bike considering your budget. At this end of the scale there's not much difference between each manufacturer, so its good advice to stick with a well known and reliable brand (Trek/Specialized/Kona/Scott)
As you BMX I'd suggest getting a fairly small frame. One of the biggest mistakes people make when buying MTBs is getting huge frames.
I'm 5'8" and ride and won't go above a 16" (seat tube).
A simple way of checking this is to stand over the bike with you feet on the ground. There should be at least 2 inches of clearance between your crotch and toptube of the bike.
If you going to be riding the trails (dirtjumps/singletrack) then a smaller frames bike will be easier to move around underneath you.
However make sure you have a long enough toptube also. As the bikes get smaller they also get shorter (which is great for the jumps, but rubbish for speed and stability). As long as you don't feel cramped you'll be ok.
The Trek seems in the pic seems to have a pretty long stem. Get rid of that as soon as you can. :) Anything above 80mm will just destroy the ride and slow down steering. Why they spec long long stems nowdays is beyond me... its such old fashioned thinking
The spec of the Trek would concern me a little, you might find yourself replacing it quite soon (ie. around about a year).
There's a nice Specialized at EvansCycles.com at a similar price. £20 more anyway ;). Better frame, better spec and a shorter stem too :D
Gimmie a shout if you want any more advice though
:)