I am new to RIA and have not been actively involved with this technology in my projects as we using server-side Java, but I want to gain some experience with RIA. My questions are:
One question to ask: Will you be building for iPhone or other handhelds that don't support Flash/Flex? It's far and away the most powerful RIA tech out there, but it won't play on iPhones (not unless you do a "jailbreak").
Other RIA tech emerging/established include jQuery, Yahoo User Interface library, MooTools, and other JavaScript frameworks/libraries that give you many of the features of Flash and play nice on the iPhone. In addition, there's HTML 5, which includes a ton of features long sought by developers. HTML 5 is used by many of the latest handheld devices.
If you're only building for non-handheld devices, Flex is probably your best bet; it gives you the best results, and because it uses a plugin object to render, is (almost) independent of the browsers' rendering quirks: Build once and have it render the same way across browsers that have the Flash player.
If you're building for handhelds and other platforms, then jQuery, MooTools, and other JavaScript libraries, along with HTML 5, will provide you with tons of features. HTML 5 will take some time to be fully supported, but many of its features can already by used to deliver a richer experience to users. Read about "progressive enhancement" and you'll build apps that have a core of functionality that meets everyone's needs while also providing cutting-edge eye candy for those using newer browsers.