It's true that some of the newest games in 2011 are going to out date your computer a lot, games such as Crysis are already more than most computers can take. The PC that I have which is severely outdated is coming to the end of it's life, it is more than 3 years old, it can hardly run games like Call of Duty: Modern Warfare without crashing due to the High CPU usage. Most of the time the game freezes when there is a massive gun fight, suddenly it just freezes. These are just some of the symptoms of old PC's, some problems are from CPU, RAM, GPU(Graphics cards), and Mainboards(motherboards). Due to the new release of some of the games and new OS's most PC's now need to be updated every few months in order to be able to run these games and that's why you shouldn't wait a while before you upgrade.
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I hope this post has been informative, and remember if your PC is freezing and crashing a lot, then its time to upgrade!