Overclocking is just overclocking, 64 or 32 bit. I just use the BIOS overclocking controls. You need some serious work to get that last bit of performance out of the system, so I just don't bother
I also am a fan of the cool-n-quiet setting, and serious overclocking stuffs that up.
With the RAM and HTT on auto, the BIOS should turn them down as necessary. I get the message that my RAM is running DDR266 at boot up, so the BIOS seems to have turned it down 2 notches for me which is erring on cautious but about right.
I run a 64 bit Fedora Core 5 install at home. It uses more disk space as you still need all the 32 bit libraries around, but is very usable for me. I gather there are some seriously fast 64 bit copies of mencoder around if you do video conversion.