A couple 2,000MHz 4GB memory DIMMs would probably give you all the performance you need, and then some.

An SSD would give you better performance but less storage space for that cost.

Personally, I'd RAID-1 or RAID-5 two or three storage drives instead of running a RAID-0 for the OS/Programs performance.

GTX 580 is an amazing card, when running single card I think I'd rather take the CUDA+PhysX benefits instead of the slightly faster AMD offerings.

Get a Corsair PSU, they have legendary customer service and generally make great products.