Lots of good answers here and I tend to agree with most or all of it.
At a high level, the system is all about BALANCE. 64-bit apps make you as strong as your weakest component. Today, that most often is the storage which is the best place to have it in my opinion.
CPU – i7’s offer a lot of value for the money, but you will see some better performance on dual CPUs
GPU – don’t NEED to have a great GPU but the difference is amazing. The Quadro 4000 is a sweet card and will last you a good long time. 5000 is kickin… GPU blog article
RAM – don’t skimp! 32GB is good, 16 adequate, 64 awesome and future proof.
bus speed – if one mobo is faster across the bus, choose it.
HD – make your boot drive an SSD, you’ll be happy you did. SSD blog article
Dennis – Adobe guy