Pretty much the only thing a 64-bit cpu is good for is...well...running 64 bit programs (and being able to have more physical ram.)
getting a 64-bit cpu means that when there are more 64-bit programs out there, you'll be able to run them
right now, dual-core is hot. The basic idea being more processors connected together can accomplish a task faster than a single processor can...but this depends on the program. Although more programs are being written to be able to take advantage of multi-core cpu's, most everyday programs like firefox, your favorite media player, etc... can't really take advantage of the extra core(s).
There have been cases in third-party benchmark reviews where a single core cpu actually outpaces the dual-core cpu
For editing, dual core will most likely do better than single core (I can't speak for other programs, but premiere pro 1.5 definately runs smoother on my athlon64 x2 3800+ than it does on my athlon64 3200+. Both processors run at 2.0GHz and have 512kb L2 cache per core)
If you get a quad-core cpu, you'll be running into the same wall that early dual-core cpus did. The fact is that there just aren't enough programs out there that can utilize all 4 cores...yet.
For "future-proofing", a quad core would be a good idea. Even though there may not be any single program that can use all 4 cores, you can run 4 programs that are meant to run on a single core (and take a lot of single-core cpu power) simultaneously.
(think along the lines of ripping, indexing, encoding and listening to music all at the same time)
The other big part of the editing equation is memory. More memory running faster really helps out with programs like AE
I don't have any experience with 32-bit vs 64-bit OS's...but if you get a 64-bit cpu you can run either.




