And you can clock 955 with the box original CPU cooler fan. To 3,5-3,8 if lucky and know what you doing.
Not with a pre-built computer you can't. OEMs do not [except in special cases like alienware] include overclocking options in their BIOS.
Also, if it's a chain built computer, it has an OEM copy of windows on it. OEM versions are one shot installs that tie to your specific hardware. I'm almost positive replacing the CPU would invalidate your version of windows. Really, by the time you're done, it's really REALLY more cost effective to build your own machine rather than trying to retrofit a cheap OEM machine.
Also keep in mind, just because your machine is AM3, it does NOT mean your BIOS will support any AM3 processor. Unless you know they made a version that came with that specific processor, it's a gamble.