can i run Radeon HD 5670 Sapphire 1GB GDDR5?

No, your CPU is not too weak. A HD7770 or GTX650Ti would actually be a good balance for it.
My earlier comment in reference to your PSU, is that even if it is only capable of 275W (i.e. it is a cheap PSU-shaped object with a liar label on it), it is powerful enough to run a HD5670. If it is a high-quality PSU like a Corsair VX or an Antec Truepower New, it would be much more than enough.