For systems with 64 bit CPU and 2 GB or more RAM. going amd64 is definitely preferable.
An exception to this is if you depend a lot on 32 bit proprietary software, this can cause a bit more hassle depending on your distro. In that case you should verify that the particular distro provides the required infrastructure for your specific 32 bit app.
Basically 4 gb ram is not that much, can be fully used by a PAE enabled kernel even with 32 bit. You can use flash 10.1 beta more easy too. However if you want that the system wins in benchmarks you have to use 64 bit. Depends on usage case, i would just install a 32 bit and a 64 distro - as you can share the home (only flash needs to get replaced all the time, but you can script that). 10 gb / for each distro + rest for /home shared should be simple.