Not really knowing what the 32-bit Windows can or can not do, probably it does not know how to use memory at above 4 GB physical address, and you are stuck to 3 GB plus some. (Which for a salvaged memory is not too expensive…)
My current home computer is ASUS A8N-SLI Deluxe with AMD X2 (stepping 02) processor, and it definitely can use all 4 GB of installed memory.
However original BIOS didn’t setup the “memory hoisting” correctly, and the end result was rather horrible crash if I tried to “hoist” and to use the memory above that limit. Then very recently there came out a new BIOS release that did it correct, and I am happy big memory spender…
So, if the motherboard BIOS setup has “memory hoisting” buried somewhere in the advanced menus, turning it on may help - or maybe not.
Then running your 32-bit operating system with /PAE enabled may let your operating system to use all 4 GB, permitting more big programs to run simultaneously without resorting to swapping.
By the way, for performance reasons I do prefer to run 64-bit operating systems, and 32-bit applications.
This is equally true for SPARCs running Solaris, as well as PC hardware running whatever.
Very few applications do benefit from 64-bit pointers, but operating system core is usually a different story, especially if you do have more memory than fits inside the first 4 GB.