If you are just referring to general usage of your PC like surfing the net, reading writing emails, watching movies, then you are not going to notice any significant differences between an AMD or Intel duo/quad core CPU that's 2.9GHz - 3.2GHz.
If you are going to be playing games, then you may notice a difference especially if you have a duo core CPU, but the game recommends a quad core CPU (like GTA 4). Most games are limited by the video card rather than the CPU as long as the CPU is fast enough. Also, games that can make use of more than two cores would play better on a quad core CPU. When comparing a quad core AMD vs. a quad core Intel CPU, it is possible to detect a difference, but most of the time you need to see benchmarks especially if the frame rates are high. Can you tell the difference between 60 frames per second and 66 frames per second? I bet most people will say no.
If you are doing something that requires a lot of processing power (like encoding video), then that extra increase in CPU speed can be worth it. If a 2.9GHz CPU takes 120 minutes to encode a certain video, but 3.2GHz CPU take only 110 minutes, then it may be worth paying for the extra $$ for the extra 300MHz if you encode video often enough.
Intel CPUs performs better than AMD CPUs at the same clock speed for both games and general programs (like video encoding). A modern quad core Intel CPU @ 3.0GHz will always beat a modern quad core AMD CPU @ 3.0GHz.