Sound system upgrade? please help!

[Peteygasp]

I currently have the S676A hi-fi sound system in my X5 e70. Being as this system is clearly not even close to sounding appropriate for a BMW, I was looking to upgrade the sound drastically.

Anyone know of the best, and most cost efficient upgrade to put in my X5???

I have read all over online and end up on other threads that don't point me in the right answer, and I have no knowledge of car audio systems myself.

Just to be clear, I do not have the stock system, or the premium hi-fi system, just the one in the middle (hi-fi) is what my car is equipped with -- (only nothing this because I have read that the premium has more speakers such as two speakers in the rear doors, etc. etc.)

also, quick question to include here, do i have a center speaker in the center of my dash with the hi-fi system, or is the mesh covered thing something else? (am currently out of the country so I can't test it out; just trying to get things squared away so I have a plan to get this done when I return)

anyway any input is appreciated! just looking to seriously upgrade this sound system (while keeping cost in mind) that sounds like something is going to explode when i put it up to full volume -- thanks!