Ok, so right now I want to play games like BF3 and MW3 and Crysis 2 at max settings on 5670x1080, then overtime decrease that resolution to 3840x1080 on a triple monitor setup.
CPU - Core i7 3770 (not unlocked, will use mobo utilities to take it to 4ghz)
GPU - XFX HD 7850 (Will overclock to a core clock of 1.2ghz if possible)
ASUS LGA 1155 - Z68 - PCIe 3.0 and UEFI BIOS Intel Z68 ATX DDR3 2200 LGA 1155 Motherboards P8Z68-V/GEN3
Ram - Corsair Vengeance 16gb (Ram has gotten so cheap over the years, it only costs $90...)
HDD - 1tb Caviar Black WD Sata 3 6 GBPS 64mb cache
SSD - 64gb OCZ vertex 4
So yeah those are what I like to call the replacement components, those are the ones that you should be replacing because there isn't a need to replace your sound card or PC case or PSU unless they break, they don't contribute to the performance all that much.
assuming you'll upgrade that GPU in the future, and there's no drastic changes to the hardware requirements for gaming following the release of the new gen game consoles, i'd say the usual 4-5 years max. 3-4 if you want to keep that GPU. more if we're real lucky.