Sorry if I came off with a temper.

Didn't mean to.
This is why I do business the way I do ....
I support aprox. 100 different customers ... aprox. 250 - 300 PC's. (And 3 POE video surveillance setups.)
Everything from animal supply stores to banks, doctor's offices, home users, lawyers, manufacturing facilities, retail stores, to veterinarian offices.
Pretty much any small business environment around that I can get a hold on.
Approx. a 100 mile radius.
I'm a "one man show".
I simply don't have time to support sub-rate hardware.
Using Systemax PC's, you can spend around $800 - $1000 and get good machines with good parts, AND off the shelf parts so you can fix them when they break.
Oh ... and did I mention AMERICANS ON THE PHONE WHEN YOU CALL TECH SUPPORT!
That's right ... built in Ohio ... and supported in Ohio!
I've been doing it this way for going on 11 years now and so far it has worked out great.
As best as memory serves ... I've had 4 PC's actually go down on me.
In summary .... (as I'm sure you already know),
Spend more up front ... have less problems down the road.
In answer to your questions ...
I've yet to try a Win 7 workstation w/a 2K server. But I wouldn't be afraid to say that it'll work w/o incident.
Intel is putting out a NAS box now that works great! I have 5 of them in place. No problems as of yet.
Replace the workstations first ... then if things are still slow, make sure routers, switches, & cables aren't bottlenecking things.
If not ... then it would be time to look at replacing that server.
P.S. In no way am I making an attempt to boast about my customer base, no doubt I'm a "small fry" compared to others!
Its no more than an attempt to validate my contempt for lesser rate hardware.