please excuse my newbie'ness.

i have an unlocked Samsung Galaxy S (original), i know it has the necessary radio etc.. for Czech, but i have a UK T-Mobile sim in my phone.
Can i just arrive in Prague and send the text you mentioned and it will allow me 500mb via the o2 network?
do i not need to get an o2 prepaid sim or anything first?
edit
whoops, think i just worked it out. can you just go into any o2 store in Prague and obtain a prepaid sim with no ongoing automatic recurring costs?