The problem is that you will need to fool around with the registry tweaks if you want to be able to load over 1GB worth of sample starts. As Martin said, check out my page regarding the tweak and download the application that does the tweak for you and create a restore point on your system before applying the tweak to be safe. To this date I haven't had a single report of system instabilities after using the original tweak and there have been some thousand downloads of the app now plus the ones who did the tweak manually in regedit. All disclaimers apply of course as this is a freebie from my side, and the fact that I don't get reports from every single user.
You should be able to reach about 80% at least (about 1-1.1GB on the msg32.exe process) on that machine.
As far as GS3 goes... Out of the box it will load more samples, but not much so you still need the tweak to get a significant increase. The 3.03 version includes a memory configuration utility to apply the tweak (the functionality is inherited and somewhat improved from my small app) and in my opinion it should work fine considering that I know what it does quite well. BUT, and there's a big BUT here, there are people who reports system problems after using this app though which make it even more important to create the system restore point first. FWIW, on my three GS boxes it has worked perfectly.
Why Giga only sees 512MB when XP gives every process 2GB of addressing space is a pretty complex story. Part of it is certainly known, but part of it is still held in darkness as the memory management functionality in XP is not fully open and publically documented. The documentation talks about XP optimizing the control values (some of which we're manually setting with the tweak) on startup and so on.