As far as the X-terminal computers a pentium class computer (actually even a 486 will work if it has a pci motherboard, it'll be slower) with a supported pci graphic card (2MB if you want to use 1024x768 resolution, many older cards will work OK as X-terms with the vesa driver) probably 32MB of system ram and a 300 - 500MB hard drive. A pentium computer with ps/2 keyboard and mouse would be best choice, just because of replacement mouse and keyboard availability.

Note: you're not going to be playing games on these, they are for internet and office type applications and for that kind of use they function very well.

As far as the X-Term OS you need a base system, networking, and X.

rock