>See you don't have to buy the newest of the newest
That's kind of a key thing when you're building a PC if you don't want to really bling it out. Since you can change individual components of your machine there's a continual arms race between the manufacturers to release the biggest and bestest thing. This is most noticeable in video cards. That constant churn is enabled by charging a huge premium for bleeding edge gear, but you don't need bleeding edge gear since the most recent games were designed to run on far less powerful cards. If you compare the specs of video cards you'll find there's a point where a moderate increase in processing speed / power.
For example
http://www.ncix.com/detail/asus-geforce-gtx-750-ti-33-94339.htmvs
http://www.ncix.com/detail/asus-geforce-gtx-760-directcu-de-85679-1396.htmvs
http://www.ncix.com/detail/asus-geforce-gtx-780-poseidon-7f-95510-1226.htmAll the same make and brand, with very small differences between the first two and an extra gig of ram and a little more power on the third, but with huge differences in price. When I build a new machine (once every 8 to 10 years) or upgrade my video card, I go for one at about the relative price range of the first one on that list. Then I use it until it croaks or I literally cannot play a game I want with it on low settings. That takes about 4 years.
The two most important components to spend lots on are the motherboard, to make sure it has plenty of upgrade capability, and your power supply (because newer, bigger video cards need more juice). If you buy those two things top of the line, or at least not out of the bargain bin, a long time will go by before you need any major upgrades. A new video card every 3 or 4 years and possibly a better CPU at some point over the lifetime of that tower, and that's it. The only time it gets expensive is when you're no longer able to upgrade with the motherboard you have, because by that point it often turns out that the architecture of that board is obsolete and you can't re-use your cpu or memory.
Then you buy a new mobo, cpu, ram and video card...then reuse the old case, hard drive(s), power supply etc.