That's not worse, don't worry. Probably the E5500 chip, bit low on cache, but not a terrible processor. However it won't run The Witcher 2 smooth on higher settings, so don't waste too much on a graphics card (GPU) - but you're not planning to
Cat's question about PSU related to the power supply unit in your computer. Graphics cards can slug quite a lot, so when you're upgrading it's necessary to check what the current unit is and see if it'll cope.
You're going to have to open up the computer to put the graphics card in anyway, so I'd suggest you open it up in advance and look at the side of the PSU (looks like a cube about 14cmx14cmx10cm at the top or bottom of the case - with your power chord plugged into the back of it) - there should be some information on a sticker - the best info is a table listing the different voltages etc. - post that info here if you can. If not, the make and model should be on a sticker somewhere.