Hi i am getting a new case and psu my computer is currently a http://h10025.www1.hp.com/ewfrf/wc/genericDocument?cc=us&docname=bph06973&lc=en#N254 I am getting a new case and a new psu what voltage psu should i get my only upgrade so far is a graphics card i am planning on more upgrading what psu voltage should i get ? also whats the difference between at and atx and what one should i get ?