As you can see, the debate rages =)
If you have a 64 bit processor you should start with a 64 bit OS, you are simply going to get better performance. If you google search you will find the benchmarks to back that ;)
Take note: most of the benchmarks are old as 64 bit has been on parity for long enough that the vast majority of people advise 64 bit (and so fewer people are publishing benchmarks).
64 bit Ubuntu has been on parity with 32 bit for several years and you are no more or less likely to have problems with one vs another.
If you do run into the increasingly rare problems with 64 bit Ubuntu you could downgrade to 32 bit.
In terms of RAM, you can use up to 64 Gb RAM with a 32 bit PAE kernel, so the size of your RAM does not really enter into the equation, except that 64 bit will user your high ram more efficiently.
Understand "high ram" is relative, but at a lower level 4 Gb is "high ram". If you wish to know the technical details, see the kernel discussions / documentation.