Windows XP + PAE + 6GB RAM: See more than 3.5GB?

Even with Physical Address Extensions enabled Windows XP still allows only 4 GB of memory. I know this can be a little confusing since Windows Server 2003 allows up to 64 GB of RAM (with the proper processor support). Sadly your OS is bounded to 4 GB in this case. The kernel may support up to 64 GB with PAE, but it is locked down in XP. It seems the only reason they still include it is for DEP support. From Physical Address Extension - PAE Memory and Windows:

Although support for PAE memory is typically associated with support for more than 4 GB of RAM, PAE can be enabled on Windows XP SP2, Windows Server 2003, and later 32-bit versions of Windows to support hardware enforced Data Execution Prevention (DEP).

EDIT: I just want to add that this 4GB cap includes dedicated video memory or memory set aside for a on board GPU.

No. You won't see more than 3.5GB until you upgrade to a 64 bit operating system. If you need to use all 6GB now, you will need to get that "eventual" Win7 64 bit install now.