OK, I bought this Optiplex and one of these Grandstreams to do testing on. I guess it will be just 1 POTS line at this point, but I will be able to get a feel for the system. I chose to use that Optiplex because people claim it has good compatibility and it's cheap.

I will probably buy two Digium TDM400P cards and 6 X100M modules once I decide to go with Asterisk. Does that sound right for my setup?

I don't know about going with the Grandstream phones in the long run. Many people here have a short temper and slam phones. They don't look like they're up to the abuse.