If you had a practical and capable machine for what you need already, I would say fair enough. But you are going to have to invest something now, and it is going to make you some money (hopefully). The MacBook will not depreciate that much between now and the end of the year, The Quicksilver will not be worth much by then, even with the CPU upgraded.
In business, you don't often have the luxury of waiting for technology that you don't absolutely need. And while Clarksfield would be nice, you can do the things you need without it.