It isn't worth the money to buy a processor for a prebuilt. The chances are that your motherboards FSB and multiplier is locked in bios and will not allow you to set the proper clock speed for the new processor. Processors change on a daily basis, and so the bios needs to be updated as such. But prebuilt manufactures really don't care if your bios stays up to date or not. They would rather you go out and buy another complete system...Your best bet is to just upgrade your motherboard and CPU, and you will need to upgrade your ram to DDR3... I would say that a good P55 or H55 motherboard an i3 540 and good DDR3 ram will give you just as good of speed as a core2 and or a Phenom2...http://www.newegg.com/Product/Product.aspx?Item=N82E16819115221&cm_re=i3_processor-_-19-115-221-_-Product 540 $120http://www.newegg.com/Product/Product.aspx?Item=N82E16813131661 H55 motherboard $87http://www.newegg.com/Product/Product.aspx?Item=N82E16820211409 DDR3 1600 $64