XF 1.5 How much memory is require?

I have a forum but sometimes for almost no reason it gets really slow to the point it doesn't even load the page then an error comes up and says connection timed out error. it also gets logged to the forum when it happens. i asked my host but they said i might need to increase the memory using the php.ini but i dont think i have access to it. so my question is how much memory does a small forum need?