I would say, go with 16GB. It's a good size, most motherboards are capable as long as they have enough slots, and Windows 7 Home Premium maxes out at that amount.
Unless you are running copious amounts of VMs or something. If you are, then by all means, go for 32GB and get Win7 Pro.
I debated this issue myself, and I decided to go with 16GB and Win7 HP. I had already purchased like 80GB of DDR3 RAM, in both 1333 and 1600 speeds, all 1.5v Gskill, during Newegg Shellshocker deals. However, they were all 4GB sticks.
I would have had to splash out for 4x8GB sticks, for like $200, and then spend money on Win7 Pro additionally. For me, it just wasn't worth it, since basically everything I do will fit in 16GB.
I was, however, happy to upgrade from my 8GB C2Q rigs, because at the time I upgraded, I was running a BOINC project called Neurona@Home, which was some sort of Neural-network simulation, and the WUs were using up 8GB+ RAM each on their own. (Thankfully, only one would run at one time.)
On my 8GB DDR2 C2Q rig, Windows 7 and the rest of my programs would take 1.0-1.5GB of RAM by themselves, so I ended up swapping a bunch on my SSD because of the N@H WUs. So it was wearing out my SSD pre-maturely.