Does this sound good for a custom built gaming PC?

Yep everything except a raid array or raid array of ssd's for rapid app/os loading times. What you gonna use 24gb for? simple gaming will never use that, unless you intend to install games to ram drive... If your buying a 3 way sli board do you intend to put a second or third gtx580 in that box, if so that power supply may not suffice, definetely not with tri sli. all the best