I own a 2005 Boxster S, and perhaps can lend some of what I have learned to the conversation. 2005 is the "oddball" year in the 986/987 progression. It's a crapshoot on the IMS bearing. It could be the late 986 style single row, or it could be the 2006 and later 987 large IMS bearing. No one really knows how many of each were made, and no one knows when and where they made the switch from one to the other. Some say the early cars had the 986 style, but there is no proof of that either. It seems Porsche just apparently pulled an engine off the rack when it came down the line, and you got whatever engine was lined up next. I would think from a common sense point of view you're more likely to have the 986 style bearing in an earlier 2005 model year build, but there's really no proof of that at all anywhere, only speculation. As was said earlier, the only way to positively prove which IMS you have in a 2005 is to pull the trans and flywheel and take a look.
It's kind of a "6 of one, half a dozen of another" type situation. If you have the earlier bearing, you can change it out easier. If you have the later bearing, there seems to be less of a chance that it'll go bad. Pick your poison, I guess.
As for getting shouted down that "THIS IS A 986 FORUM!!!!!!!", just ignore. It's usually the same 3 or 4 guys. The rest here are awesome and helpful. Enjoy and PM me if you have any other questions on the oddball 2005. I won't shout at you, I promise!
Good luck with your purchase. I love my '05. It's a great car.