So you ask yourself, UBUNTU 32bit or 64bit?

The 32bit/64bit memory usage (ie 32bit cannot use more than approx 3GB) is true for all OS’s (it’s a physical thing) but there is a thing called PAE (physical address extension) which can allow 32bit processors to address upto 64GB of memory.
If you install 32bit Ubuntu on a machine with >3GB RAM it automatically uses the PAE version of the kernel and allows the use of all your RAM.

To do the same in windows you need to use the Server versions ($$££$$).

To run movies etc you will need to have installed the restricted extras as mentioned by pbpersson earlier (this is to do with copyright issues).

For flash issues first try the Flash-aid add-on for Firefox, it sorts out a lot of common issues.