I would recommend clean installing a new OS and new Postgres on a second hard drive. You would then need only migrate your DB out of 8.4.0 and into the new Postgres. Then once completed this new hard drive is swapped in to be your new boot drive. This would leave your current production setup functional and running up until the switchover. I recommend you upgrade to a SSD drive for this as well, it will speed up that Mini. (i did)
or something like that.