Yeah big difference dude
Tri channel is for the x58 chipset, i7 are the only CPU at the mo that supports it
Dual channel is for everything else basically
You will need a dual channel kit for your p55, I recomend going for a 1600MH kit if you are thinking of overclocking, somthing like the corsair dominator 4Gb kit.
For cooling I would go with a venomousX with two fans in push/pull
Also, I think the mobo your dad is getting is an E-atx board, basically bigger than normal.
I don't think the case you have chosen would house that size or board, if I am wrong som1 please correct me.
If that is the case, you will need a bigger one, somthing like the haf 932.
And tbh, the antec 900 is oldschool now, maybe go for a 900-2 or 1200
I kno its a lot to take in, but I hope it helps you dude
Just ask anything you need to kno and we will try and help