One of the features of Snow Leopard is most software is now fully compiled for 64 bit Intel. By default most system still boot in 32 bit kernel but with Intel Xserve and Mac OS X server the default is 64 bit. This strategy to to maintain best compatibility we kernel extensions (kext). A USB serial adaptor driver is a good example.

The servers and Mac Pro's "generally" don't have these client side add-ons so we feel it of best benefit to boot the 64 bit kernel. There is also some intelligence built into OS installer which examines drivers to see if they have 64 bit version to set default first boot.

Good article here on snow leopard and 64 bit...

This Apple kbase describes the addition processes that can run with run 64 bit and adding RAM. Mac OS 10.5 doesn't scale with RAM.

Other articles related to 64-bit....

Which machines boot 64 bit kernel by default.

http://support.apple.com/kb/HT3770

How to switch to 32 or 64 bit kernel.

http://support.apple.com/kb/HT3773

Screensavers need to be 64-bit.

http://support.apple.com/kb/TS2913