I've been studying for Cisco for the last few months and ran into this question right away.  Sort answer, it is not possible, as I'm sure you've come across in other searches.

Basically, the Catalyst hardware is proprietary, so it can't be emulated using software.  Router hardware can be emulated, which is why GNS3 (dynamips) only needs an IOS image for a fully functional environment.

As you say, the best bet is to get a router IOS and use the NM-16ESW module (selectable by the Slots tab in the router properties).  I am using the 3640 12.4-25d image (I don't think all IOS images support this module).

In case you felt compelled to hack a solution (as I did), a few hours of failure prompted me to contact a network engineer (6xCCIE!) who confirmed this is not possible and that NM-16ESW module was enough for him until he got to the CCIE level and then he just purchased equipment from ebay.

I have a fully functional test environment of my network here in case you have questions on setting up GNS3.

Looks like my 2621, 3745 and my 3660 image all support NM-16ESW. I did not realize though that would give me sufficient coverage for the exam. I should have some 3750 G POE switches to play with soonly but unfortunately will not have them before I do my CCNA. I'm sure they will come in handy for CCNP studies though.

Thanks for the links Sil, had not seen those blog posts. I'm having a lot of fun with GNS3 and have some really evil ideas for connecting it on live networks.  Now if I can just figure out how to handle all the traffic those evil ideas will shove through my box (GNS3 tutorial I saw indicated it could only handle 1/100th of the traffic that a normal router can)

I'll be revisiting this thread later with my experiences. Thanks again for the info Sil and Yatz.