1) building a pc through dell is building it through is dell.com/alienware
5-6)1200$ is a great budget to start off of, i built my system off of about 800$ w/o peripherals, and if you love computers, it would last about anywhere from 2-4 years, well thats what "us enthusiasts" say.
7)upgrading is easy, its like putting together puzzle pieces with a map
8)by overclocking, you get more from what you originally payed for; with they are TONS of guides on this site and you would learn with a little research.
8a) by overclocking itself, it requires patience, so yes practice is a must.
9)folding is an enthusiast's game/sport to rack up numbers out of hardware, but theres guides all over this site too.
9b)maybe if you do something completely wrong......... like installing it backwards