unless you screwed up superfetch, Windows 7 natively takes advantage of all the RAM. I assume you have 64-bit. with 32-bit obvioulsy it won't use more than 3.2 GB.
I think if you use Revit MEP or os, 8 GB is recommended and 12 GB probably fine. but for everyday user,it won't matter. the HDD is more a bottleneck. an SSD really speeds things up. But with superfetch and 12 GB, it already should be OK.
for new PCs with quadcore (assuming you actually use it) 8 GB is recommended, so 12 GB is not such a waste of money if you got a good deal.