80+ i a power efficency rating. How much power if draws to get its rated power. 80+ Bronze is 82% efficent. higher efficency ratings mean several things.
1) Lower power bills. a 400W PSU and a 400W 80+ PSU will both supply 400 W to your system, the 400W PSU, at say 75% efficiency, will use 533W from the wall to supply it. the 80+ one will use 500W from the wall. and an 80+ Bronze would use 488W
2)better quality: To make the units efficient, you get better quality parts put into the unit, meaning less chance of failure that can take out your system
3) less heat: Because they are more efficient, the power supply generates less heat, making cooling your system easier
Really, I don't see a point in home building a computer when you can get an equivalent one from dell for ~$300. And at least Dell gives you some type of warranty and they put it all together. Another options is buying re-certified computers from newegg or buy.com
Plus -- most important parts will have a 3yr warranty (or even 5yr in some cases)
Ubuntu has improved tremendously in the last few years. I would say it is more similar to a Mac OS... Once you load everything you need -it is quite easy to use. Particularly if you are not using graphics-dependent software