we're almost getting to the point where it's justifiable to upgrade from a core 2 duo and a HD 3950 with 2 gigs ram.
Now that the ps4 hardware is announced, we just need to wait for microshit to announce the xbox 720's specs, and then wait for one of them to actually be released. And from there, the wait for newegg to offer a package deal for a factory new mobo + i5/FX and then a HD 5/6XXX card.
Otherwise, once those two are released, you won't have to upgrade again until the ps5 and competitors are released. As this generation of console games shows, PC games are ports of the console, and both MS and Sony are trying to make their consoles work more like desktop computers so it's both easier to make games for their consoles which means the computer ports aren't going to be gta4 levels of terrible-console port.