The more time passes, the more serious this problem is. I believe that somehow DreamHost has been lowering the memory thresholds more and more, until at this stage even a fresh, clean install of WordPress will almost always give 404 or 500 errors, even with zero plugins installed. From "mildly annoying" (last year) it has begun to become "a constant nightmare". Well, I use CloudFlare on top of everything, which will minimise the 404/500 errors somewhat (since 40-60% of all traffic will come from CloudFlare and not hit DH's Apache webservers), but during PST "prime time", it's almost impossible to completely avoid all those errors.
I wonder if this gets worse with PHP 5.3 FastCGI. Some tests I've been doing show that WordPress with PHP 5.3 FastCGI and Page Optimisation almost always use close to 256 MB of RAM, while other software — like, say, phpBB or even Joomla — under PHP 5.2 (not fast) consume far less, e.g. 100 MB or so. Comparing different software is hardly fair, but I wonder if there are some recommended settings to keep WP memory consumption low. And please, don't simply advise "delete plugins" or "use simpler themes". As said, I have been getting 404/500 errors with a fresh WP install with just the TwentyEleven theme.
I wonder if turning Page Optimisation or Extra Web Security off will consume less memory? Also, caches reduce bandwidth and CPU, but sometimes at the expense of using more RAM. Has anyone made a few tests to see what combination works best?