I work in this computer shop for the last 10 years now. One thing that I have been noticing with games is every 3 years or so, gaming specs will peak. And after that, some specs will be the current requirement or standard.
Eg.
1998 - Unreal. Okay, if you did not of purchased a Voodoo 3D Accelerator for this game, you most likely did shortly after.
2002 - I think it was Battlefield 1942 that really caused a lot of people to purchase new video card. Lost count the number of times I had this conversation.
Customer :- I need a new video card.
Harper :- Let me guess, you are playing Battlefield 1942?
Customer :- How did you know?
2004 - Far Cry, Doom III and Half Life 2. That was a very good year for video cards and upgrades.
2007/8 - Crysis and Alan Wake. I do see that in 2008, we will have the death of APG as every one that purchase a new gaming rig with a APG video card in 2004 just to play Doom III will start to realise that the current games that are out now don't really cut the current game.
And most people have the trend of purchasing a computer system now
Then upgrading the video card 18 months to 2 years later.
Another 1 to 2 year later, they will be replacing that computer system.
One thing to keep in mind with game devs, it not just a game they are selling to the public. They are also selling the gaming engine to other devs so they can create new game. Create a new gaming engine is a not of work, and not all programing house have that sort of skils. So for the next few year, be are most likely going to be seeing games using Unreal Engine 3 and Crytek Engine (etc etc). The engine devs cash in the sale of their gaming engine, and in the mean time get to work on creating there next one to be released in a 2 or 3 years time.
So with that being said, and based on current trends that i have seen, you could most likely pick up a new system now with out having to worry about another Crysis coming up for another 2 years.
