I'm idling at 29C while web browsing right now. Don't think I've ever seen it over 70C when gaming thus far. Stays pretty quiet with the default fan settings too.
If you plan to get a higher resolution monitor then it could be worth the 4GB. As of the games out right now nothing needs 4GB at 1080p. The extra memory will not give you better performance. It will simply cost you more.
Year from now. Two years from now. That could change. But right now there's no real need for it.
Sitting here at idle I'm 36-39C on different cores. While gaming with say, Farcry 3 or Battlefield 3... I don't look very often but I'm probably 54-60C. My CPU is also overclocked from 2.66Ghz to 4Ghz though.
No worries. Just buy a good CPU (if you don't have one already) and use a good aftermarket cooler on it. The stock fans (that come in the box) aren't very good for constant CPU loads if you're gaming a lot. They're meant for people to browse the internet and watch movies. When gaming the generic cooler will allow most CPUs to run fairly high temps.
Although some video cards can get pretty dang hot (GTX 400 series would run 80-94C when 100% load) a CPU should never go over 70C.