How many people are going to be hitting the site? I cannot speak for FileMaker, but Apache will spawn a httpd daemon for each connection. Each daemon will consume a tiny bit of your RAM.
Are we talking 10,000 page views a month? or 10,000 page views a day? If you are running off of an iMac, I'm guessing it won't be that heavily abused.
Running your FileMaker Database will be more processor hungry than Apache, but i bet that running both you will want MUCH more than 128 MB. Plus, $100 now will save you some headaches later.
I would recommend AT LEAST 512MB.
Also, don't forget that OSX itself will consume about 60 - 70 MB's of whatever RAM you have installed. If it doesn't have enough RAM, you will start getting page-outs, which will affect your hard disk performance, which will in turn affect your database performance.
FaRuvius