SuperPi is also a 32-bit application, so it can only see up to 2gb of memory, which is exactly how much memory SuperPi sees in that case. There is something else going on here though. If trinity is running 32-bit OS, the GPU could be mapping 1.5gb for video which would only leave 2.5gb for everything else. Then SuperPi once again, only sees 2gb because it is still 32-bit.
I was hinting that the user saw an 8-ish figure and guessed it's the total amount of system RAM when those are only 8MB of memory used by the app. It's easy to mistake bln with mln, and end up to GB when thinking about it.
As to your suggestion, here's my competition: that 32 bit system actually has only 2GB of RAM. 3Dmark sees 2GB, SupertPI sees 2GB... it's not hard to figure Trinity ran on 2GB of RAM.
Oh, and UMA can't be larger than 512MB. The BIOS/UEFI doesn't allow it and by default it takes up 256MB or 512MB if large amounts of RAM is detected. So... no, it can't share 1,5GB for video.
On the 64 bit system... SuperPI isn't able to detect the amount of RAM, mainly because the variable that stores that value is of int(32) type and its max value unsigned is 4,29 bln, when it needed a double to store the value of 8 bln bytes, the amount of RAM the Llano system has. It has nothing to do with actual type of app, even if it's 32-bit, it still relies on the os core functions to get those values.