I personally run servers that run Apache, PHP and MySQL, hosting probably a dozen different sites with around 12 databases for each, ranging from Blogs to Forums, etc.
Right now, one system is using 1.5GB of memory out of 2GB of memory, probably a good 90% of the memory being consumed currently is split between mysql and apache. 1/3 of the 1.5GB is cached. Let me also mention this is a DNS and Postfix server as well.
So telling me 256MB is too much, I fail to see without any further details and I think you're wasting your time switching to lighttpd for something that probably isn't an issue to begin with.