Cheapest 3-4TB RAID Storage Solution?

I've got a 2TB eSATA drive that is full. It's a Hitachi Deskstar 7K2000 32MB cache 3.0GB/s

I need more space (1TB more would be plenty) and more redundancy because if this drive crashes there will be over a hundred hours of work encoding my DVD and Blu-Ray collection for my DLNA server.

I want to spend as little money as possible. Running on a mid-powered Dell Desktop running Vista as my server (it's my wife's machine so I never bothered upgrading it to Win7 - Vista has worked well despite past horror stories.)

I'm guessing a 3-bay eSata enclosure combined with a RAID eSata controller card in the PC is the best/cheapest option? This PC is about 3+ years old. Maybe my best bet is a used PC off Ebay that already has a RAID 5 3x2TB array?

I would assume the RAID config utility should allow me to take the existing drive combined with two more blank 2TB drives and convert to RAID 5 without losing the data on the existing drive?

I just checked what controller I had bought as it was a while ago. It is a Silicon Image SIL3132 2-port controller and it supports RAID. Hopefully the SATARAID5 utility will do the job?

Maybe the very cheapest would be to just get a 3TB drive and partition it into a 2TB and 1TB partition then mirror the original 2TB drive and use the 1TB extra for non-critical data? That seems a little hokey though even if it is possible.