I agree with shortstuff's choice of memory. It's an excellent deal and a good match for that setup.
Windows 7 seems to be faster than Vista. However, once they add in the security code it will probably be the same. I guess we'll see...
All right, you definitely don't sound like you'll want THREE video cards. That 850TX is a good choice for you. How to decide if you want a second card: play with just one and see how it feels. You can easily find benchmarks for the most popular games by Googling for reviews. For example in Crysis you can expect 35 fps at 1920x1200 with a single GTX 285.
http://www.guru3d.com/article/gigabyte-geforce-gtx-285-review-test/13
With two, I suspect it will be around 50 fps. Most games will be just fine with a single card even at 1920x1200, because most games are less demanding than Crysis.