Buy an amp and then upgrade the speakers once you have the money to do so.
I have a Sony STR-DH800 which is brilliant. Alternately look at Onkyo or Denon.
Speaker-wise, for a cheap 5.1 set maybe start out with Tannoy SFX and then move onto bigger things when you see fit. I personally run a Mordaunt Short Alumni 5.1 setup which, for the money, is epic.
Best advice I could give is not to skimp and end up wishing you had spent more in future (especially on an amp).
For more info have a browse on avforums, they have a pretty good classified section too if you want to go second hand.