I have a dedicated server running about 30 websites. One of those sites has become a problem due to some custom coding and server configuration requirements. I was wondering if a 512MB Linux VPS would be suitable to run a single PHP website with MySQL, but no email. This is a moderately active ecommerce website.

Try it out. If it doesn't work theres a 30day money back guarantee. At that point it should be able to be migrated back to your dedicated server.

That's really impossible to answer. I've seen mysql/php websites run off of 256 meg VPS's fine. I've managed a PHP site distributed over 60 web servers getting 13 million unique visitors per month. We'd probably need a little more information on the traffic than 'moderately active' (which we could have very different definitions for) to give any real advice, and even then there's so many other factors (the code, what kind of caching, what kind of db optimizations, how many queries are doing sequential scans vs how many are being indexed, etc) that we couldn't give a real sound recommendation.

My recommendation really would be to ask you how much the client is worth to you. If one month of poor performance could cost you the client, then go with the 1 GB VPS, and if it's not taxing the server at the end of the month, move it down to the 512 meg vps.

The client already decided to go with 1GB.

Thanks for the feedback though.