By what i've understand till now it can be used as an extra force to the d5 square when playing against the sicilian...But sometimes just crumps my position..So probably it's not always a recommended move...What could you advise about it? :O

I think it can be good against the Boleslavsky hole pawn structure, stopping the d5 break.

http://en.wikipedia.org/wiki/Pawn_structure#The_Sicilian_.E2.80.93_Boleslavsky_hole

This is hard to say. It makes a lot of sense to play Be2 to support g4 and h5 in many sharp lines. In the Dragon, and especially the Yugoslav proper, Bc4 is played to stop an easy d5 yes. 

In the Boleslavky and the Najdorf structures with d6-e5, you can see it too. If black is slow to develop, a well supported knight in the centre can be very annoying. 

Regardless of all that, there is a line called the Sozin or the Fischer against the d6-e6 pawn structure. The idea is somewhat different here. It is more likely white has intentions to place a sac on e6 or thereabout. If black unsuspectingly carries on with a6,b5 Bb7 or even Nbd7, blocking Bc8, he might be in for a surprise. The line a6,b5, Bb7 is normal, but it needs to be played accurately. 

A general rule in chess is that all general rules in chess will have significant exceptions. ;)

Bc4 can be bad:

- If black has prepared the d5-push with e6
- If black is ready to push b5 
- If black has discovered attacks along c-file
- If black wants to play Ne5 or Na5 anyways
- If a knight on b3 is blocking the natural retreat. 

Bc4 can be good:

- If d5 can be effectively prevented. 
- If you can target e6 and f7 with for example f4-f5-push.
- If you can anticipate b5 or d5 by Bb3.

Some Variations:

- It is really bad usually against Taimanov or Paulsen setups. (e6 and a6 played very quick.)
- Bc4 is common against the Dragon as black has allready played d6 and is not ready to push b5 yet.
- Bc4 can be played against Najdorf or Scheveninger in combination with f4-f5 quick assault.
- Bc4 can be played in classic GPA if black has played d6 early on.
- Bc4 is the move in Anti-Sveshnikov. (Italian setup against e5/c5/Nc6) 
- Bc4 will be recommended in some Morra-Gambit or Alapin variations. 
- Bc4 is a good move in some ultra-sharp variations like open O'Kelly Sicilian. 

Well..i think all of you(especially Turm_), completely and very well-posted, answered the question and thanks a lot for that.. c ya around... Scott as always with his laconic way lol :p