If you're going to use only office applications then it's better to go with dual-core processors like Core I7-620M or I5-540M, than with quad-core Core i7-720QM. Your programs probably won't use all cores on the i7-720QM, and for single- and dual-threaded tasks dual-cores are faster. From the page below you can see that with/without Turbo Boost the i7-620M has higher or much higher clock frequency than the i7-720QM:
Also, CPU is not the only factor in determining whether new system will be faster than the old one or not. Amount of RAM, type of hard disk and model of video card are important too. Well, in your case video card is not important. Make sure you get as much RAM as you had in your desktop system.