If you do not need any control panel, I think 384MB would be enough to host wordpress or drupal site.
Just remember to optimized the MySQL configuration and apache if you use it.
For the OS, whether Ubuntu or CentOS, both are best OS for web-hosting.
I'd go for Debian or Ubuntu as apt is easier on the memory than yum.
128MB should be enough for a barebones setup but will also leave you with enough breathing room for experimenting with other stuff after you're done setting up your hosting stack.
For OpenVZ you might want to consider purchasing a plan with twice the memory.
Tuxlite.com Complete LAMP and LNMP script for Debian and Ubuntu.
Given that this is a "play & learn" VPS, and will probably be a low-memory VPS (according to your other thread), here's a few suggestions:
1. Stick with 32-bit.
2. Try Debian 5 (or 6 if the provider offers it).
3. Learn how to install Apache, PHP, MySQL (via the package manager).
4. Learn how users and user home directories are created.
5. Learn how virtual hosting is setup. Eventually you're gonna host more than one site
6. Get a functioning website that serves PHP...
7. Keep notes
8. In the VPS control panel re-install the OS to CentOS.
9. Repeat steps 3-7.
10. Decide which feels more comfortable: Debian or CentOS.
11. Re-install the preferred Linux distro (if required), and get the system back to step 6.
12. Move on to installing & configuring Wordpress, etc.
13. Look into optimizations for Apache, MySQL.
14. Learn how to monitoring the VPS for performance, load, etc.
15. Investigate alternate webservers like nginx, lighttpd.