I've been following various forums in regards to VPS performance. I noticed a lot of people were ditching Apache to use lighttpd or nignix (spelling?). While I dont anticipate my projects to attract thousands of visitors per month, I still want to be able to have an environment that's stable and doesnt slow to a crawl should one of my apps take off. I've read instances where RAM consumption was close to max when running SVN or when an apache process gets out of hand.
I dont plan on having any control panel so that should help with RAM, but I do plan on having SVN, mysql, php, and apache. How much abuse can a VPS with 512mb of RAM take?
SVN is probably the most significant factor in your searches. If you have many trees or large trees (especially if coming from CVS with lots of history), you'll end up needing LOTS of memory else performance will be poor. VDS and VPS often charge a premium for setups with lots of memory, so you might find it worthwhile to find some company that'll make a deal on a dedicated machine with, say, 2 gigs of memory. Otherwise, you might want to look into colocating a machine of your own so you can buy a motherboard which supports 2 or 4 gigs of memory. Memory is cheap, but getting memory in a server still comes at a premium.
If the price difference of a 512mb VPS and a low end dedicated server is close then go for the dedicated server. I would think a 512mb RAM VPS would handle all that you need to start with and if your project attracts more visitors then you can move to a dedicated server.
In my dev environment, I have a VPS running subversion (/w Apache mod_dav_svn) and it doesn't use more than 70mb of RAM. I allocated only 256MB for it and it runs without any issues.