You know as much about this as I do. I'll describe what I did - with improvements for my "slips" - FWIW. (I do have a degree in photography, and one in computer science.)
My target is usually about 85% on a cost/effectiveness basis. Beyond that it is just cheaper to wait 6 months and buy a better base machine for $600 instead of $2,500 today, as you mention. Plus, I over obsess and overengineer when designing, but on a day-today basis, that extra 5% to 10% is totally irrelevant.
So, I built a core network of 4 machines with 2 gigabit lans. One for "local" data - photoshop macgine, server, nas - and one for "external" data - wirelress lan, laptops, etc.
I would suggest 2 machines as a start:
1) A Photoshop/Lightroom **only** box that may not even have a daily ionternet connection! Probably an i7 920, and
2) An application/data/storage server for **everything** else - any core applications, data storage, offload processing, printing, backups, etc. Probably a Dell Poweredge quad Xeon 300 or 500 series, no highg-end video required. With dual gigabit network cards and a hardware Raid controller (a 3rd box would be an actual NAS, in addition to this server.) Base cost $500 + RAID card + RAM + disks.
In the photo machine, I personally would put 3-4 320GB WD 7200 drives in Raid 0 or similar for OS. Apps, and Scratch disk (2 or more separate logical drives of course. As many as you can squeeze. Cloned for instant re-install) I am ambivalent on SSD - a bit too pricy yet. (I got the WD Caviar Blue for $30 each. They area great size for OS, aps.)
Then a couple of 1 to 2TB HDD's - one for RAW files (local working copy) and one for WORKING files (lightroom database, etc.) Backed up to the Ap/Data server often over gigabit.
For RAM, I'd do as much as I could with 2GB chips. The 4GB chips are just too effing expensive! Might as well buy an extra box instead for $600!!!
My 3rd box is the NAS with 6TB and Windows Home Server. It is off 90% of the time.
My 4th box is my "everythinhg else" machine -a dual core laptop with 4GB for watching TV, music listening, e-mail, Outlook, etc. I keep the server as a pure server (Running Hyper-V: was ESXi from VMWare.)
Everything else - bluray players, tv's, 3 other laptops, etc run off of the "external" lan to avoid traffic. There is actually a bridge to a 3rd router, which is the internet gateway for all.
Just thoughts. Have fun! (Sorry for typos, hand pain ....)