My host says I need to increase the memory size in PHP.ini ... how do I do that?

I have a store runnin 1.38. It's been running fine for a year so, the othre day, I could not load any product pages. Everything else loads, but product pages.
I contacted my hosting and they told me to increase the memory size in PHP.ini

Here's a sample of what I'm seeing in the error logs for this issue:

[Mon May 04 12:40:09 2009] [error] [client ] FATAL: erealloc(): Unable to allocate 491520 bytes, referer: http://www..net/index.php?main_page=index&cPath=2&zenid=
[Mon May 04 12:40:10 2009] [error] [client ] FATAL: erealloc(): Unable to allocate 491520 bytes, referer: http://www..net/index.php?main_page=index&cPath=2&zenid=
[Mon May 04 12:40:10 2009] [error] [client ] FATAL: erealloc(): Unable to allocate 491520 bytes, referer: http://www.*.net/index.php?main_page=index&cPath=2&zenid=

You may be able to remedy this by increasing the PHP memory limit in your php.ini file.

Can someone assist me with this issue? Thanks in advance.

When running into a memory limit problem you can adjust it by using:

memory_limit = 32M

Keep doubling it, to a max of 256, until no more errors.