Why? Theres no real reason to spend the money on the CPU upgrade really.
If you insist, the problem with Dell is they're famous for using proprietary components. So nothing I say should be taken as a guarantee.
Yes the 3550 should work on that motherboard without issue.
As far as Windows7, its pretty forgiving when you do system transplants, so there shouldnt be a problem, but generally speaking, you should always do a clean install for best results.