1) Ram only affects FPS in-game by a very small amount, by about 2-6FPS, so it's really up to you to decide.
2) Both are great CPU's the 955 is probably the better value for money.
Also AMD are very close to launching the new "Bulldozer" CPU's it might be better to wait a few weeks so you can get the latest and greatest.
3) YES!, a third-party cooler is essential to getting a decent over clock.
4) Overall it is a decent system and should perform well on most of today's high end games.
Hope this helped out