Guys stop giving crap advice like using a GTX 460 (!) for a PhysX card */facepalm* when there are very few games (Like Mafia II) that actually benefit from a dedicated physx card.
OP Just stick to the plan and get those parts. I might swap out for the 650w PSU though as one guy suggested. It will do fine for GTX 460 SLI.
GTX 460 SLI gets you better max frame rates than a GTX 580 and is very overclockable, so I'd go wtih it.
http://www.anandtech.com/bench/Product/314?vs=305
It is also more power efficient as you can see here. Btw be careful when listening to some of the ridiculous power estimates some people make. If some of these nuts had their way you'd be buying a high quality 850w power supply to run a single GTX 580.