Some miss information floating around:
Windows memory manager controls the access to physical memory via working sets, most programs cannot touch physical memory, and in fact virtual memory can be larger that 4GB;
By setting the 3GB switch in the boot ini file for certain configurations of your system then SU can have 3GB of memory( Less what some device drivers use.) vs the normal two while the kernel gets reduced to 1 GB. I was able to open an ~350MB SU file ( Window Xp sp3, 4GB Ram) using this approach. Does not mean speed was great.
For the 64 bit systems SU is still limited to the 4GB Address space because it runs as a 32 bit app. However the operating system can remap memory to use RAM greater than 4GB so the app. can have the total 4GB. Your BIOS needs to be set correctly to max use of your 64 system. Make sure to read the manual.
For those interested read these articles http://blogs.technet.com/b/markrussinovich/archive/2008/11/17/3155406.aspx and http://blogs.technet.com/b/markrussinovich/archive/2008/07/21/3092070.aspx