Most people say that a cpu upgrade is impossible but the true is that you can upgrade most computer up to 1ghz. Cpus are kind of expensive but it works the investment, the computer perform much faster if you just increase the cpu speed by 0.3ghz on new systems imagine what a big different if you change the cpu from i3 to i5 or i7 there would be a huge different, the pages will pop up faster and you will be able to multitask.
The 2 most popular cpu brands are amd and intel, both have a variety of cpusto fit the different kind of clients. If you ask me what do i prefer? I would go for intel because amd cpus overheat a lot but with a good cooling system you would be just ok.