You can check how many slots you have in your computer by opening it up and looking directly, or going to http://crucial.com/
and running their detector. They recommend RAM that works, but usually its overpriced.
Installing new RAM is pretty easy.
Use the picture here for references to what the RAM looks like - http://sonic840.deviantart.com/art/Compute...r-1-7-111402099
Unplug everything from your case.
Hit the power button again just to make sure its off(And uses any remaining power sitting in the circuits)
Open it up. I can't really say how exactly how to do this, as it is different from case to case.
Again, depending on your motherboard and the layout, the RAM is in different places. Usually its somewhere in the middle of the case
You should have at least one stick of RAM already in the slots. If you get two pieces of 512MB RAM, and you only have two slots, you'll have to take that one out.
To remove any previous RAM, just push on the two white(may be different colors) button-like ejectors on the sides.
Pull out the old RAM. It should come out easily (I'd recommend keeping it just so if one of the new sticks of RAM go bad at least you can put that old stuff back in and still have a computer.)
To install the new RAM, just firmly push down the new RAM into the same place you took the old RAM out. Just make sure its aligned right.
Make sure the RAM is in all the way, and the button ejector things are up next to the new RAM. Repeat if needed.
Thats all. Just put your computer back together, plug back in everything, and it should work. While starting up it may detect it and say it in the startup once.
Also you can take a vacuum in the case and clean it if you think its really dusty in the case.