A TH2Go isn't cheap, for sure, but you should be able to get one for around $300:
http://www.google.com/products/catalog?q=triplehead2go&btnG=Search+Products&cid=3059127492681558920#ps-sellers
Being a niche product, I expect higher prices. If they became as commonly used as mice, the price would drop significantly. With my current gaming budget there is no way I would have one had I not won it in a contest.
TH2Go does support ATI cards. AFAIK, the only major limitation is a lack of 5040x1050 @ 57Hz, otherwise ATI cards are good-to-go. (ATI owners please correct me if I'm wrong).
Windows XP does have multi-monitor support, but does it have the ability to treat 3 screens as a single display? I thought it lacked that feature.
I don't play CS:Source, but
this thread seems to indicate that TH2Go is allowed.
It sounds like you have all the right hardware for SoftTH. Have you tried it? That would give you a good taste of Triplehead gaming.
Bottom line--if I had the money to spend, I would have grabbed a TH2Go right after I learned about it. Now that I have one...I refuse to go back to single-screen gaming. :D