Knowing what the clients will use it for would help. Does each need client 500MB of memory, 1GB or more? That's something that you can determine up front. LAN speed should not be an issue as it works fine on a fast enough WAN.
It's for a middle school, the kids will basicly do math programs, reading programs and word, excel, powerpoint, with shared photos and documents. thats pretty much it. so 512mb of ram would probaly do