Differences

The P55A has SATA III as well as USB 3.0.

Thanks for your answer!
Will the lack of SATA III affect me?

I wouldn't worry about it, unless you are planning on spending big bucks on an SSD in the foreseeable future... It will be a long time (think - years, not months) before physical hard drives will be able to take advantage (other than caching, of course) of the SATA3 bandwidth; there are - count 'em, exactly two SATA3 SSDs that could take advantage of it, from Crucial, but they don't seem to have their firmware 'together' yet; and ,so far as I know, the Marvell SATA3 ports are unable to handle TRIM in RAIDs. When you see Intel using SATA3 in an ICH, then it'll be time to think seriously about SATA3!

I don't plan on buying ssd nor adding a second gpu...so is it gonna fill my gaming needs and overclocking if I ever decide to go that way? Is there a chance it will hinder my performance? Sorry for asking so much, but I'm kind of paranoid because it's the first system I build by myself...

Nah - the Intel ICH's are faster 'n the Marvell SATA3's anyway.. All the 15/17 CPUs are mighty overclckers - pretty much all you need is aftermarket cooling - and you needn't go nuts buying top-end parts - likely any aftermarket cooler will get you close to, or over, 4GHz...