Well, first off, I will not pretend to be an expert in this area but I can say that I have a bit of experience with graphic design programs, I've taken a few classes and have gotten some good insight from my teachers. From what I know, most 3D design software such as Vue6 xStream and Maya can see at least four cores, if not more. And with a budget of $2,000, I would say go for a nice quad core processor and the ones that seem to be at the top of the market now performance (and unfortunately also price) wise, are the Intel Core i7s. Given his line of work, I believe this would be the best suited for him. As for the GPU, ATi and Nvidia are both fantastic companies, and I'm not too sure about more recent cards, but I know ATi used to have some solid cards designed specifically for workstations like this. You'll have to look into workstation graphics cards to find out if they continued with this... and I'm a bit of an ATi fanboy so I will always recommend them first. They offer great performance at a cost that is usually a bit less than Nvidia. My favorite motherboard brand that I love and trust is ASUS, never had a problem with the ones I've purchased in the past or the one I'm using now, not to mention that most of them have overclocking options that will automatically overclock your components for you if your brother would choose to do so. I guess I've been out of the loop for awhile storage-wise, I wasn't even aware that SATA III was around yet, but I don't think that the actual rendering of files such as in Photoshop and in 3D design software will be affected by the storage speed, but saving large files might be, so I say go with the fastest that you can, which I suppose would be SATA III. Also, with graphic design software, fast RAM and possibly a solid amount of RAM will be necessary. Make sure your motherboard supports faster RAM such as the DDR3 1600, and I would say a minimum of 4GB should be sufficient, depending on how many of these programs he has open at once. I have 4GB and have been able to keep Maya and Vue 6 xStream open together and running smoothly. If your brother runs into problems, go for more, maybe 8GB.
Sorry about the lengthy reply... if you don't feel like reading it all, I'll try to sum it up for you without my personal experience nonsense intertwined in there haha.
Recommendations:
Intel Core i7 processor, ATi OR Nvidia card (check out the charts on this site for good comparisons and price checks), ASUS for a motherboard, fastest storage speed you can get (probably SATA III) and possibly a fast drive (at least 7,200 maybe even 10,000+ RPM), and at least 4GB of the fastest RAM you can get (probably DDR3 1600).
I wish you and your brother the best of luck with your build!
-Jordan