Another severe case of "fear of numbers". Any reason why you didn't supply model numbers, or do you believe that Belkin and Dell only make one model router and laptop?
No, you don't need to buy everything from one manufactory. That's what standards like 802.11b are all about. Anything with 802.11b scribbled on the package should work.
If your "new" Dell P4 Notebook really is new, then one of the stock wireless card would probably be the "best" choice. See: