Computers, especially desktops seem to last a very long time nowadays. I built one over 4 years ago for under $800 that is still going strong (i5 2500k, 8GB). Added an SSD and upgraded the video card to a Radeon 6950 in the last couple years and that's all I intend to do to it for a while. It's still super quick, especially with the SSD and can play newer games: Skyrim (maxed out /w high def textures), Battlefield 4 (nearly maxed out) for example are no problem. The very newest stuff might give it some trouble but honestly not really concerned.
To me, the one upgrade that is really worth your money is the SSD. They are so affordable now and make enormous improvements to how the computer actually performs in day to day use. I would definitely sacrifice in other areas to get at least a small or medium size SSD.