One of the biggest reasons that switching to a Linux VPS could go wrong is not enough memory. Most hosting providers start the packages at 128 MB of memory. When running a web server, a MySQL database server, and PHP, 128 MB is usually not enough for the average user. For a web server with that setup to run smoothly, you will need at least 256 MB.
Add your visitor count and a statistics package; maybe even a firewall as well as anti virus and anti spam for an e-mail server. You would be better off choosing an overhead between 512 MB and 1024 MB.