It depends on what you are talking about for upgrade. Hard drives kind of depend entirely on failures of other ones or just plain needs at the moment. That being said the best thing that you can do for your computer (gaming or otherwise) in terms of an upgrade is to get an SSD. That shit is the difference between night and day. No more "turning your computer on" and waiting for it to start. I used to go to the bathroom in the morning when I started, go in, open a browser, make breakfast, then start my day. This was on a gaming rig. I upgraded to a SSD and you turn it on and it simply is on. The thing that takes the longest is the Windows logo because, it turns out, that is on a set timer. Once you are in windows you are in.
Video cards I try to go every two-two and a half years depending on the cycle of new stuff coming out at the moment. I go for about a really nice mid-tier card that is hopefully on sale at the moment.
RAM, almost never. DDR3 has been the standard for almost a decade (think about that) and if you hit it right 8 gigs is about 30 dollars for a no name brand.
The Intel i series of processors have been really good. A friend got an i7 when they first came out and he is just now thinking about upgrading. That is about five years for a CPU.