LGA 775 CPUs are very easy to install and there are a number of video tutorials on Youtube and the like which show you how to do it properly.
And yes, the CPU will be good enough for any game, and the performance you get will depend far more heavily on how powerful your graphics card is. A very fast CPU paired with an abysmal graphics card guarantees rubbish performance compared to a slower CPU combined with a fast GPU, assuming the latter isn't bottlenecked by the slower CPU in any way.