More shared_buffers instead of effective_cache_size?

Hi,
I have a virtual server with 256 MB of RAM. I am using it as a
webserver, mailserver and for postgres. So there is something like 150MB
left for postgres.

Here are my configs (I haven't benchmarked...)
max_connections = 12 (I think, I will not have more parallel
connections, because I only have 10 PHP worker threads)
shared_buffers = 24MB
work_mem = 1MB
maintenance_work_mem = 16MB

(effective_cache_size = 80MB)

Normally, the file-cache is part of the free ram. But on my virtual
server, it looks like if there is one big file cache for the whole
hardware node and I do not have my own reserved cached, so it is not
easy to find a good value for effective_cache_size.

I've also benchmarked the file-cache using dd (100MB file)

1. Read from HDD:
104857600 bytes (105 MB) copied, 8.38522 seconds, 12.5 MB/s
2. Read from Cache:
104857600 bytes (105 MB) copied, 3.48694 seconds, 30.1 MB/s

That is really really slow (10 times slower than on my other machine).

What would you do now? Increasing shared_buffers to 100MB and setting
effective_cache_size to 0MB? Or increasing effective_cache_size, too?

Thanks for help.

Regards,
-Ulrich