So, UserVA is good for not exhausting kernel. /3GB is actually really bad for servers that have lots of inbound connections that use TCP, as one example.
http://blogs.technet.com/b/jeff_stokes/archive/2008/08/28/pte-depletion-handle-leaks-and-you.aspx
I wrote that a while back to talk about some of the balancing act of /3GB and USERVA. Clint Huffman, a buddy of mine, wrote this as well, but it was on a system with 16GB of RAM so might not fully fit. But it might help understand.
One trick I did to my daughters SIMS3 binary was to change the header so it could use more RAM. This likely isn't necessary here, but it is a cool stunt if you run into the need.
This is a good forum post on RAM too http://forums.anandtech.com/showthread.php?t=1900898
Anyway back to the SIMS 3 story. http://simswiki.info/wiki.php?title=Game_Help:Insufficient_Memory_Crashes
Check that out. Specifically the part on XP. ANd this too
http://www.nexusmods.com/oblivion/mods/23254/?
"The tool will add the "LARGE_ADDRESS_AWARE" header for you to your executable."
So basically, in 32 Bit land, you can hack a binary (after you /3GB your boot.ini and userva=XXXX to balance it out) to make it LARGE_ADDRESS_AWARE (its just a bit set at compile and is easily flipped with a hex editor) and then the binary will be "3GB enabled" and use more RAM