OOh it is not as simple as that (gaining the old sites PR that is). PR is also gained from internal links, so if the old site had 20,000 internal pages many of them with backlinks, then unless you can snag those backlinks you are going to lose the link juice. Also Google is zeroing expired domains that change hands and change Ip's & Content. so you might have bought a pig in a poke.
I bought 4 test domains and in every case where I changed the ownership details they were zeroed (apart from the one that had not expired). The one that had not expired kept its pr4
The easiest way is redirect match, or a straight 301 redirect catch all. BUT this will dump everything on the home page.
My advice to you is to get a list of all the backlinks pointing to this site, and set up pages with the corresponding urls (even if they are dynamic, then just recreate them manually). then you can be sure you are keeping the link juice, and transferring them back into your site.
Potchy yes, but secure, as when the bot comes along, it will not see ALL the pages re-directed to the home page it will see the pages that people have 'voted' for still existing
