New processor

Your Current configuration looks pretty good.But your graphics card is only 256 mb memory.So i think you need to buy just a good memory card.

Do a full upgrade or just increase the ram. before everything check for any virus or spyware because they tend to eat your cpu more than regular software programs.

I think you have sufficient configuration of your computer.. but if you want to upgrade your system then you may try Intel i7 processor .. it is latest technology.. and your speed of system will be good after trying this processor..

I think the issue is with your processor and not with the whole of your computer. The processor which you are using i.e. AMD Athlon is quite old and kinda out dated now and also its slow. So you should just change your processor. You can upgrade it to processor like intel dual core, core2duo, i3,i5 etc depending upon your budget as well as your need. And your RAM and graphics card is sufficient and shall not be a problem once you upgrade your processor.

There are many factors that can make a computer is slowing down. Be careful with your decision since it can be just a waste of budget. I used AMD Athlon 64 3000+ 1.8 GHz processor, ATI Radeon HD 3600 Series with 1 GB Ram and it runs pretty quick. I can play Crysis, NFS Shift and other hardcore games without any problem with my computer. Compared to mine, your computer should be much more faster since you used nVidia graphic card -it works better with AMD processor than ATI Radeon- and you even used 2 GB Memory Ram. I suggest you check your power supply device first if it has its performance decreased and unable to provide the sufficient power input for your computer. It is easy to know since usually it will make your computer over heated. You can also tweak your system to reduce its workload and speed it up a little. Too bad you didn't mention what is your motherboard device specifications since it can affect your computers performance. O, almost forgot, a damaged hardisk can be badly slow down your computers performance too.