i want to upgrade . what all can i add without straining cpu?

Add RAM. 1-2GB is about average. If your gaming maybe a better graphics card than the onboard. A sound card wouldn't be bad to upgrade but not very neccessary.

From cheapest to most expensive.

RAM

Sound Card

Graphics Card.

you really don't need to upgrade the sound card. We need to know the speed of the AMD Sempron and your other specs. I'd say you can put in a video card and not strain the CPU too much (an 8/9 series from nVidia would probably be too much)

RAM always helps in any situation.

need to know what type of expansion slot you have. i.e. pci-e or agp.

and what type of ram your motherboard accepts.

edit and add what type of computer you have and or what type of motherboard for more help.