You're probably having to restart because of caching. My OpenACS site generally only comes down for software upgrades. I've got two sites, actually, on the same server with 256MB RAM. They each end up using about 40MB RAM for cache when they've been up a few hours. After that things stabilize.
If memory leaks were significant sites like aol.com would fall over every few minutes, so keep things in perspective. When was the last time you heard that aol.com, or digital city or other large aol sites, were down? Given the general dislike of aol in the geek community, we'd hear of failures just like we're treated to every little outage or problem at one of MicroSofts sites.
If AOLserver seems "sluggish" it is probably because of its memory consumption and the resulting swapping. Feed it enough RAM and it's anything but sluggish. I've seen lots of derogatory comments about AOLserver in my day, but I've never heard it called "sluggish" before.
Caching, of course, does have its upside which is why AOLserver's so aggressive about caching things. It's overly aggressive if you have a small machine, but heck Fry's has 256MB PC133 ECC memory available for $99 this week. Caching by both webserver and the database engine is important for web performance, and memory's cheap.
You might also want to read Tim Perdue's article as to why SourceForge switched from MySQL to PostgreSQL, as he has real experience with both databases under conditions of high load (which is why he switched).