I don't believe you need to upgrade the server, as that should be plenty to run what you have. And the only way you're going to benefit from more memory is moving to a 64-bit OS since you already have 4 megs.
The slow queries look like they are definitely causing some issues. Not sure if we've touched on this yet, but the best place to start is with a couple things.
Sas your MySQL server been optimized at all? Also with the number of posts that you have, you probably need to look at the individual tables as well if they haven't been optimized lately.
I'd grab mysqltuner and run it if you haven't done so. This will at least tell you if there are any variables that are severely out of whack and how many of your tables are fragmented.
