Car Sound system? I have a 2002 bmw 525i and i want to get 2 pretty good subs?

I have a 2002 bmw 525i and i want to get 2 pretty good subs in the trunk but i know nothing about anything of this sort. Do i need to upgrade anything else on the car to get the subs? How much would it cost to get them and have them installed (most likely bought and installed at best buy)? Is there anything that i should also get or need to get?Car Sound system? I have a 2002 bmw 525i and i want to get 2 pretty good subs?around 450