You no longer need flexbuilder as flashbuilder can be used to create projects using either the flex 3 sdk (flashplayer 9) or the flex 4 sdk (flashplayer 10).
Unless there is a reason to go back to the mx based flex 3( for example if you need to target flashplayer 9) it is best to go with the newer features you get with the flex 4 sdk(this is flashbuilders default).
Flex 3 used the Eclipse framework but had some integration issues. These are cleared up in FB4.
FB4 introduces the Spark framework for enhancing the user inteface, and even comes with some nice templates and many more online...build your application first, then skin it with free pre-made skins, or some very professional ones at a reasonable price.
As mentioned FB4 uses the Flex 4 SDK which is faster, cleaner and has additional features.
Check out a whitepaper on it.
I thought FB4 was just going to be a disaster and a waste of time, but I gave it a go and in my opinion Flash Builder is awesome!
I found FB3 rock solid and I loved only having one name space.
FB4 is quirky to say the least.
Only reason I don't switch back to FB3 is because FB4 changed the way states are done, and that alone has saved countless hours. I love the way STATES are being handled in FB4!
CrazyMerlin, you got a url for the FB4 skin site. wow that sounded x-rated...
The adobe skins can be found here: http://www.adobe.com/cfusion/exchange/index.cfm?s=5&from=1&o=desc&cat=328&l=-1&event=produ ctHome&exc=15
I have found this site to be good for little snippets, but there are more out there...I just get a coffee and follow the links:
where to get the X-rated skins anyway?