This is one question since depending on what operating system, applications and other stuff you have running may have an impact. Secondly, what kind of internet connection do you have as if you are on dial-up then broadband may improve some things. Lastly, consider adding more memory if that is where there is an issue since 512 MB may not go as far if you use programs known for being hogs when it comes to memory. It could be the hard drive but there are a few other possibilities too...
I have computer with processor AMD ATHLON 64*2 3600+ ;512mb ram ;but my computer lags what is the problem is it with hard disk which is uata