RAID 1 and Existing PC

I have a SOHO (peer – peer). Last week I had one of my workstations go down. Now I need to do two things
a) purchase a new computer
b) Add Raid 1 (disk duplexing) to main computer

My question is… should I purchase a new computer with built in RAID 1 controllers and reformat the main comptuer. Or is it possible to add RAID 1 (duplexing) to an existing PC and not have to reformat and reinstall on existing EIDE drive? So new drive would update with image of existing drive and then have same data from there on out?

ALso if you know of any vendors you would recommend if possible.