My solo league is 69 years in and seems to run fine. I delete boxscores, gamelogs and messages every year
after I backup the league.
Important note, after deleting the messages, delete the messages.dat file to avoid having blank messages when you open the league.
I also move the Almanac folder to my external HDD with the league backups.
Warning; back up your league before doing this.
There is a thread somewhere that 1998 Yankees posted a couple of bat files that help to automate this. I can't find it via search.
Edit. That's with 2gig Ram and Vista.