If its an ultrabook the cpu might be directly soldered onto the motherboard. If not, most modern laptops are using sockets for their cpus. I did replace my old c2d T5550 with a c2d T7100 in the past. Make sure both cpus are the same socket. And BTW, 580m or 480m are your best choice.
why my best bet should be 580M, i see both 580 and 640 use the same socket, BGA1288, PGA988, http://ark.intel.com/products/49652/Intel-Core-i5-580M-Processor-(3M-Cache-2_66-GHz)http://ark.intel.com/products/49666/Intel-Core-i7-640M-Processor-(4M-Cache-2_80-GHz)
but CPU-Z detects my CPU package is 989 rPGA, which is confusing.