1. Upgrade your linode to something around 2 Gig for now.
2. Start playing with your MySQL or Postgresql config (assuming you're not already using redis or something similar, in which case just throw more RAM at the box). Google performance settings for your DB of choice and look for relevant posts to the memory you have.
3. If there's anything cachable, install memcached and implement it.
4. Purchase a second linode and move your database to it.
5. Upgrade each VPS as needed.
6. At this point you get into fancy scaling and there are loads of options. You should have plenty of breathing room by this point though and can plan it out.