Ubuntu :: Processor Type For New Computer Build?

A processor is very important to me and I'm curious what processor to get for a new computer build. I will be using Ubuntu for most things except iTunes, HD movies, and a few low spec requiring games on Win 7. I am building it as right now I have a half year old netbook that can do most things just fails at game play and movies but it's alright. And I have a desktop with just Ubuntu that's about the same quality but 8 years old. I want to build a computer that will be very fast for the next 4 years enough to get me to college and buy a nice laptop. I am on a small budget of around 500- 750 dollars. I have most things picked out but processor has me stumped. To me processor is a bit tricky to replace and I'd rather get a higher-end one to start and stick with that than buy another CPU two years down the road.

I don't mind upgrading RAM, GPU, and or even the HDD at a later point but I want to get the CPU as close to perfect as possible. First I heard AMD processors tend to be a bit hot. I was wondering what cooler is $50 or less that does a great job at cooling a AMD AM3 socket CPU. I do plan to overclock a very small bit perhaps up to .3 GHz plus max. First I was thinking a nice core i5 at $200 and never really checked anything else. Then, I was thinking a hexa-core at 2.8 GHz I think but decided too much and didn't need six cores. The AMD socket type also supported USB 3.0 which I instantly fell in love with. Then I found a quad core Phenom II x4 black edition at 3.20 GHz for around $160.I liked the core unlocker and it looked incredible. Now I'm rethinking a bit again.

I found a dual-core that is same thing as the other AMD one above except well only two cores. I think that would do me fine I'd I'd save $60 by getting 2 less cores. With the things I use a computer for (Ubuntu, Win7, HD quality playback, minor gaming like Runescape, and small multitasking like openoffice a browser or two and perhaps some pictures, game or movie.) I also want to be able to do this quickly for the next 4 years without a hiccup or at least to a minimum if possible. I really don't want to upgrade CPU once I get that bad boy in there. I also want it ready for higher versions of Windows like Windows 8 is supposedly coming sometime in 2012 I've heard. I just want the best CPU to be fast and efficient, run cool, and work well for me for the next 4 years.