Sue retired and we increased the number of machines, Clive!
Intel Core2 Duo processor, 2GB RAM, I'd suggest a dedicated system drive, say 80GB, plus one or more others. A 256MB graphics card should be adequate for your needs. Aim to get an e-SATA option if you can. DVD/CD drives as required.
Don't skimp on the power supply, and looks for a high-efficiency cooling system, maybe Akasa 120mm case fans.