Make two lists of components, one the really good one and the second with one of two suboptimal components and ask the builder which he would prefer for your application. If they can't find the shortcomings for your purpose, they are no good. For the rest, trust their reputation.
List 1: i7-930, GTX-480, Velociraptor, 4 disks, etc.
List 2: i7-980X, HD5970, SSD, 3 disks, etc.
If they can't see that list 1 is good and list 2 is not, they are in the wrong business for your purposes.
Well it seem to me that we built at least two systems recently on this forum that were $3k targets. Read those posts and educate your self and then submit the final list here for comments. Leave the builder a some room to pay for his services. As you can see from the true confessions it does pay to work with a semi-local builder.
I have always done things a little bit differently from Harm's suggestion, but do see the value of that.
I provide a list of desired components, and then we work over each one. I try to be perfectly open to suggestions, so long as the builder can explain and justify his/her choices. Now, I have had some very good builders, and the most recent one has proved to be excellent. I usually end up with about 90% of what I spec'ed. and then allow myself to be talked into the other 10%, with proper justification. Though this gentleman builds mostly heavy-duty medical 3D imaging rigs, he understands NLE and still imaging very well. As he is constantly building, or upgrading the hospital's/research institute's machines, he is able to stay more atop the hardware improvements, better than I can - even with Harm's wonderful input.
Now, before this gentleman, I found myself between builders. One had moved far away, and the other had left the business completely. I did about what Harm recommends, and quickly eliminated four builders in a hurry. One said that XP-Pro could not use multiple CPU's (claimed that only Windows Server 2003 could do this), while another said that only SLI would work for Photoshop. Yet another did not understand why I wanted so many large HDD's. Gone from the list in a hurry. Same complaint when I wanted 4GB RAM - he would only build with 2GB, as XP-Pro could not use even that fully.
Good luck, and when you find the right builder for you, support them, so that they will be around, when you build that next machine.
find one that knows Adobe and builds systems specifically for Adobe.
gaming computers/office computer do not = Adobe systems.