Will 256mb Be Enough

I'm considering VPS hosting for a web application, which is a typical PHP/MySQL web app, but with some extra code modules for XML parsing and full-text search etc. These modules will surely have an overhead (as compared to a simple data-driven web app) once the web app starts handling concurrent users.

Looking at various VPS plans out there, I was wondering if 256mb RAM will suffice to cater a web application described above. Besides this web app, I do not plan to run anything else on the server (so server processes will be quite restricted). But I certainly don't want the server to crawl down (due to CPU and RAM usage). Thinking about it, 256mb "sounds" too small a number for a server, but hey I'm new to VPS anyways

--
More about the web app: Technically, the website I plan to launch is a typical multi-user PHP/MySQL web application, but one particular PHP script (a search engine script which uses complex database queries and stored procedures) will be invoked very frequently (can't predict how frequently as yet) for database processing (keyword search) and returning some results back to the client.