Ubuntu 32 bits vs AMD64

[Vincent Arnoux]

Hello,
I have used before Ubuntu on a 32 bits computer and it worked like a 
charm, but since I installed the 64 bits version I am quite disappointed...
I miss a lot of applications (codecs, Mono, flash plugin, ...) and can't 
find new ones I would like to try and that are not on the 64 bits 
repositories but on the 32 bits ones.
Here comes my question:
- Are 32 bits applications working on 64 bits (I always can ask...)
- Can I force my apt to look at 32 bits instead of 64 bits repositories ?
- I really need to use the 64 bits specificities of my CPU (for my 
professionnal apps), so can I somehow install Ubuntu 32 bits and 
"emulate" a 64 bits Linux without loosing too much performance (I heard 
about Xen...) ?

Thanks for your help,
Vincent