In my case, upgrading to 11.10 (which you have to do before you can go to 12.04) totally screwed the system. It did not detect the card properly and gave me a blank screen on startup. I did a fresh install of 12.04 and now everything is working fine. I actually re-installed my 11.04 first and configured everything. Then I did a fresh install of 12.04 on a separate partition in a multi-boot config. Then only after 12.04 was fully up and running I made it the default boot option and it is happily chugging along. In case I come across any problem with 12.04 I can simply switch back to my 11.04 partition which works just fine...

If you do want to (or have to) go the fresh install way, follow this nice step-by-step guide which will make it a breeze to set up your new system as you can just copy and paste in your terminal one command after another, relax as the system configures itself. The guide is for xubuntu, but works fine with ubuntu 12.04  ---   https://bitcointalk.org/index.php?topic=82685.0

(BTW, all of the above are 64-bit versions)