When I built my PC around May-July last year I built it so I could overclock if I wanted to down the road. Games don't really push it to hard, especially if I turn some settings down. The reason I built it with Oc'ing in mind is that when it got older I might be able to get 6 months to a year extra out of it till new hardware arrived, but you need custom coolers really so after spending out for that is it really worth it?
Pros and Cons please my specs are
CPU: i5 3570k on standard cooler
RAM: 8GB 1600Mhz Corsair Vengeance
GPU: Sapphire Radeon HD 7850 2GB (This will be oc'ed first if anything)