Just my experience, but with clients i would suggest going out and purchasing prebuilt models from dell/hp or someone else. The reason is you want the warranty that comes with those items. Especially if they are getting a server, you really should be purchasing these items from a manufacturer that offers a year or so warranty (parts). If you put some mut PC's together, while you will be saving them some money (not much after purchase of legit OS's) you run the chance of REALLY delaying their purchases if you have to replace a part or two because either you fried it OR it malfunctioned. After installing an OS (at this point Vista probably) you won't be saving your client much money at all when you have to go out and purchase 300+ dollar Vista CD's.
From a large vendor you can probably get a sweet deal as well, and could possibly get a printer (laser) thrown in there and could possiblty get some discounts on a few monitors (assuming they need those).
I would say any dual core processor above 2 Ghz with 2 gig ram would do fine for CS3 running Vista Home Premium. As for the server it really depends on what it needs to do, if it is just a file server then you won't need a very powerful machine at all.