Ah, that information greatly influences the answer. By the way, I previously owned an HP computer with the single core version of your CPU (3500+). Step #1: You'll need to seek the fastest & reasonably price CPU that supports the AM2 socket and chipset; ati radeon xpress 1150. Step #2: would be to Ebay query that CPU. Be sure to check off options: used & sold. The sort all the sold items by price, to get an idea of what you'll pay; I prefer to buy items like this via the Buy It Now feature. I'd probably consider paying between $5 - $20; which is more than most people would spend for such an upgrade. The second thing that I'd consider upgrading is the GPU. It would be great if the PCIe X16 would support a GTX 750 Ti (low profile) GPU, so that you could use it in a future build. I'm not entirely sure it will. Were I buying the 750 Ti, I'd purchase new from NewEgg or Amazon, so that I'd have the option of returning hassle free if it's not compatible. If I was purchasing an earlier generation GPU, then I'd definitely purchase used on Ebay. Step #3: What OS are you running? If Windows ME then 2 GB is more than sufficient. If Windows XP, then 4 GB would be ideal, if you can find something very cheap on Ebay.
google query: ati radeon xpress 1150 cpu support
Last question: What activities are slow on your computer? What motivates you to want to upgrade in the first place.