Upgrading pre-built PC's can be tricky since some of them use proprietary or custom-made motherboards. This means you may be limited in terms of which CPU's you can upgrade with. If you are a gamer, you should get, at the very least, a dual core CPU with a clock speed of 2.8GHz. or greater. If you buy a prebuilt system that allows for AMD's AM3 CPU's, I would go for a AMD Phenom II X2 550 Black Edition Callisto 3.1GHz
. AMD's CPU lines are split between the Athlon II and the Phenom II. For users who want a more robust CPU, the Phenom II's should be considered. The Athlon II's are meant for low-cost builds and don't have some of the features of a Phenom II. The old Athlon I's (a.k.a just Athlon) are older and not as efficient as the newer ones, let alone the Phenom II's. That specific Phenom II is Black Edition so it can be easily overclocked. Intel's lineup is more complicated in that they have the Core 2, Celeron, Core i3, i5, and i7, most of which uses different sockets and require different motherboards.