I have good PC specs, but I cannot play games on a higher resolution, WHY?

first off your graphics card is a bit weak...get at least a 8500 GT...also your RAM is dead slow. at least upgrade to 800 MHz or even 1066 MHz. that will help speed things up...also on the GPU make sure you get at least 512 MB of on board memory, if not get 1 GB, it will make all of the difference in speeding up your game play. good luck