actually 32bit is 4gb and with WOW32 they can use a bit more than win7 x86 (which defaults to /2gb not sure if you can even turn on /3gb mode).
32bit is 4gb if they play some programming games with the OS, yes. I know of no games that actually do that, do you?
Games use 2GB per right now.
/3gb was used in win2003 x86 to give 3GB RAM to app and 1GB to system. along with AWE you can rock 16GB of ram in 32 bit mode which is quite handy if you have licenses for old win2k3/sql server and you want to bump the speed up (YES both modes work in ESXi!). server licenses don't really get upgrades nor does sql server so you can understand the costs of maximizing old tech that you come into with your job. going from 1.8GB SQL process to 12GB with some system cache reserve or to run apps like log shipping - sweet.
Not sure what any of this has to do with "I'm using it for gaming only" the guy is not building a SQL database machine, he's building a gaming machine on a budget.
For what he's doing 8GB, even if cheap, is very likely throwing money away for a gaming system. Gaming systems typically see pretty short upgrade cycles, and on budget machines, $30-40 saved from memory put towards the video card can be pretty significant if you're looking at video cards in the $100-150 range, for example. $40 can mean close to double the GPU power in that price range.