Install 64-bit Ubuntu or 32-bit?

Plus sides:

• It will be slightly faster. While 64bit vs 32bit apps don't make a huge performance difference, developers can make assumptions about speed-related features being present in 64-bit processors that they can't with 32bit apps.
• You can use all 4GB of RAM, as opposed to roughly 3.2GB. (There are ways around this for 32bit systems, but it's not simple)

Negative sides:

• While nowadays you won't find many apps that just don't work on 64bit, there are some (Adobe Air comes to mind), that do require more effort to install.

I've been using 64-bit Ubuntu for about 4 version now. It's my primary desktop and there's not much I haven't been able to do with it.

Adobe has a 64-bit Flash player - only for Linux - that you can download here: http://labs.adobe.com/downloads/flashplayer10.html