Wow! This community rocks on toast....
I'm now imagining bright shiny servers powered by dilithium crystals or something!
Seriously, we're looking at the technical configuration of a bank of servers. One of them connected to the outside world, the other two devoted to specific tasks (I'm not a technical expert myself - leaving that to others - but phrases like "a PHP procesing machine and a database machine" are being thrown around right now).
I second ThirdWizard's recommendation on the DL385. Good machine. The DL380's also good (AMD for the former, intel for the latter). Make sure you get the latest generation.
Also, I recommend SAS drives over SATA. SATA drives have a higher failure rate (cheaper cost...). There's not many drive bays on the DL38x, but disks are big anyway... Setup as a RAID5 or better (RAID 0 gives no protection from drive failure).
You may want to rethink your server arrangement. I find that seperating functions across servers (DB server, web Server) is nice and clean, but the network acts as a bottleneck between the web sever and the DB server. Putting both DB and PHP/APache on the same server usually gives top performance for single page requests. Splitting work across servers is best done for load balancing and redundancy reasons.
I assume y'all are running linux, apache, PHP5, and MySQL or PostGRES. We've found that makes a pretty fast application server where I live.
Good luck.