PostgreSQL: Recommendation size for shared_buffers
I would avoid placing MySQL and PostgreSQL on the same server. They compete for the same resources. If you can, port everything to one RDBMS. My obvious choice would be PostgreSQL.

Then you can set shared_buffers to something like 500 MB and effective_cache_size to something like 1.5 GB. Be sure to read hints in the manual.

But I would also recommend to add more physical RAM. 2 GB is not much. Hardly enough for good performance with millions of rows. A few more GB of RAM shouldn't cost much.

If you have to stick with your setup 250 MB for Postgres seems reasonable. If MySQL has three times as much traffic, less might be better overall. Like 128 MB.

Basics for performance optimization in the Postgres Wiki.