DonNagual covered the "No".DonNagual said:Yes and No.
It matters in the sense that a higher watt PSU is going to create more heat in your system. Other than that, no other major concerns really. You can use a 600W psu in a tiny system. It will only use what it needs. It doesn't mean you are using 600w all the time, it means your PSU can put out up to 600w when needed.