Actually, win 7 home premium 64bit supports up to 16GB(a limit set on that version by microsoft) and 128GB for more expensive versions( again a limit imposed by microsoft). Echoing ram usage, 8GB is plenty for most gaming and light work, but heavier workloads/heavy multitasking can make use of more ram. If you happen to play minecraft, it can use as much ram as you can throw at it.
As others have previously mentioned you need 64 bit to get pass the 4 gB (Video ram + desktop ram) limit.
You would not have any actual benefit for having huge amount of ram unless there is such a power hungry application ( like autocad inventor).
If you are using 64 bit for gaming then 8 gB is enough also Four 2 gB cards will work much faster than one 8 gB card (as they work at same frequency, so of you use up more than 7 gB then your pc will work 4 times slower for one 8 gb card if you have quad channel).
Also try to fill up your slots that way you can use the dual/triple/quad channel benefits.
I agree with the above posts. If you're going for gaming, 8 GB of RAM will be more than enough with a good graphic card. If you're using any type of CPU hogs like Autocad / Photoshop / Maya ...etc, it will still work fine.
I suggest you get 12 or 16 Gigs only if you're rendering, other than that, it's a waste of money.