I have a Phenom triple core 8450 CPU, and I bought a Deepcool gamma archer (suitable for the socket AM2+). Would it be a problem that the heatsink doesn't cover the whole CPU ?
I doubt that that cooler will be able to cool your cpu, its worse than stock, but if you don't have the stock copper/heatpipe cooler then its all you got.
Remove any overclocks and unlocked cores if you have any.