I have been following the usage of JavaScript for the past few years, and with the release of extremely fast scripting engines (V8, SquirrelFish Extrene, TraceMonkey, etc.) the possibilities of JavaScript have increased dramatically. However, the usage share of Internet Explorer coupled with it's total lack of support for recent standards makes me want to drop a bomb on Microsoft's HQ, as it creates a huge amount of problems for any website.

The game will need to be pretty dynamic client-side, with animations and other eye-candy things, but not a full-blown game like those that run directly in the OS using DirectX or OpenGL. However, this might be a little stretch for JavaScript and will certainly feel extremely slow in Internet Explorer (given that the current IE engine can be hundreds of times slower than SFX; gotta see what IE9 will bring), would it be better to just do the whole thing in Flash? I know this means requiring the plug-in AND I have no experience whatsoever with Flash (other than browsing YouTube :P). It also means I can't just output directly from PHP, I would have to use XML or some other format to pass data to it (JSON is directly integrated in JS and PHP can deal with it easily).

Another idea would be to provide an alternative interface just for IE, though I don't know how (ActiveX maybe? or with Flash, then why not just provide it to all browsers) or totally not supporting it and requiring the use of other browsers, although this is plain stupid from a business perspective.

So here am I, wondering what approach to take and thus asking for your advice. How should I build the client-side? AJAX in all browsers, Flash in all browsers or a mix (AJAX for "modern" browsers and something else for the "grandpa": IE).