GDC is definitely the event for investment capital/publishers. But be aware: most publishers expect you to make appointments with them in advance since their people will only usually be in for one or two days of the conference and they will be in non-stop meetings. You will not find publishers by walking the floor.

Also be aware that I have found paying the extra 600$ or so for the talks is a bit expensive; many of the talks are more geared for beginners. If you are an experienced game developer you will probably not get much out of them (my algorithms guy kept coming out of the theory ones telling me "man what a waste of time"). Some of the stuff is also misleading; we were under the impression that the MRM talk last year was going to go over Hoppes MRM article; it instead turned out to be an hour of listening to people trying to sell us a 6000$ library implementing the algorithm.

E3 is loads of fun. If you have never been I strongly recommend going; it will blow you away the first time you see it. It is really hard to comprehend the flash/money thrown into that thing (and the cute models :-) )...

But...E3 is only good for spying on new games and technology. It''s main purpose is for publishers to sell X # copies of their game to big retailers...