Building is always cheaper than buying if you are able to do so.
You have to decide how important a pc is to you and how rich you are. If you spend 8 hours per day on a pc and everything runs through your pc, such as gaming, forums, films, and so on, it's worth getting the best pc you can afford, but if pc is just an occasional event, consider your alternatives.
Personally I think i5 is borderline, and you might want i7, but it depends on your budget.
My own pc cost about 2k with monitor, mouse, keyboard etc, and is still smoking all games on full, and my card is a MSI N580GTX lightening, which is better than a standard 580gtx by a long run, but now the series is on 9, so in 3 years, that's how quick things change.
Only a percentage of the new generation is necessary though, and a huge percentage is just churning stuff out to make money, so buying something half decent is key.
As long as your power supply is good enough, and your motherboard expandable, you can upgrade individual components as and when necessary, as long as the new stuff fits in your case, and is compatible with everything else (you'd need to google that adventure).
Missus just got home so need to go offline in a bit, but can chat about it no probs.