I'm just waiting for my CPU then building a new PC.
It's a little late for this question now, but here goes.

I've got Seagate 200GB (x2) PATA HDD I'll be mirroring on the onboard Promise 20378 controller. If the controller or board itself fails, how can I retrieve my data? Can I just plug 1 of the HDDs in any other PC or will I need an identical controller to read the configuration file (if there is one?) from the drives?

I know that with Compaq controllers for servers, you can just plug in a new controller (same or 1 version back) and read a config file on the drives to access the data.