My Mining Rig - Help Please

Hey guys, i know theres threads like this everywere so im sorry in advanced.

I have never mined before and i just went to tiger direct and canada computers. Tiger direct was out of everything mining related yet canada computers had everything in stock.

280x = 449$
290 = 649ish$

Just bought there last h81 pro btc mobo.
http://www.canadacomputers.com/advanced_search_result.php?keywords=pro+btc&x=0&y=0

Now im looking to build a 4 - 6 card rig.

Here is my build list, im basing it off a thread here https://bitcointalk.org/index.php?topic=438542.0

CPU - http://www.canadacomputers.com/product_info.php?cPath=4_1210_65&item_id=063172
RAM - http://www.canadacomputers.com/product_info.php?cPath=24_311_312_612&item_id=052429
HDD - http://www.canadacomputers.com/product_info.php?cPath=15_1086_210_212&item_id=064727

Now i plan on going with the LTCrabbit mining pool. You basically download the LTCrabbit OS, it includes all drivers, configurations and scripts.

So i have done alot of research and i need some help from the comunity. I will be building a case with angled aluminium with risers.

So here is where i need help and i again apologize to the community for asking.

Since il be using LTCrabbit OS (USB bootable), will i need a hdd? my guess is no but please confirm.
I wish to go (6 x 270)/ (4-6 x 280) / (4 x 290) What PSU(s) will best suit my rig? with room to possibly expand.
I have researched my mobo doesnt need powered risers. Does this board need powered risers?

Thank you everyone for your help, i value each and every suggestion. Please, if possible, give me what you believe would be better or cheaper for my rig.

Thanks