2x360, 1x540 or 1x360?

Well, if you're actively swapping, you need to tweak more, or get more RAM. You can probably add RAM without moving up a plan.

Moving MySQL off to its own linode will make it much happier. If you are going to be spending more money, another 360 will give you more bang for your buck than a single 540.

If you're hitting swap, then you're already taking a huge performance hit. The secondary cost to that is if you're swapping, you have little to no memory available for caching disk reads. MySQL really benefits from disk caching, especially in read heavy applications.

Speaking of caching… if you do decide to stick with your single 360 (or even if you go with a second 360). Have you looked at other caching options? You can take a big chunk out of your memory usage by caching at a couple different levels in your stack. Look at using memcached with Lighttpd and/or MySQL, and some sort of op-code caching for your PHP stuff (I don't run PHP, so I don't know what the options there are).

~JW

You're missing a poll option: Upgrade to a Linode 14400 :)

I'd say either go with caker's suggestion or upgrade to a 540 plan and start caching stuff. Whichever is more financially available.

You may also want to investigate how much RAM that Spamassassin is using. Content-based filtering can compete for memory and CPU with databases and http servers. A Linode I work on takes care of nearly all spam just using sanity checks and greylisting. A DNS blacklist is another low-resource option to consider, but I haven't found it necessary.