What changes if adding a different bus DDRam to a laptop

According to Dell, there're two SODIMM slots, each of which supports up to 4 GB. These would both be on the same bus. I suppose you mean having one 4 GB 1600 SODIMM and one 2 GB SODIMM 1333. Normally they have to be the same speed, so have you checked to see if this will work at all? If you're going to add RAM, why not just get another 1600 SODIMM? If I were doing it, I'd add a 4 GB 1600 SODIMM and be guaranteed of a performance boost.
Getting back to the situation you proposed though, assuming it would work, the answer would depend on how much paging you're doing. That's a function of how many tasks you start. If you only have 3 or 4 at a time, you could well live with 4 GB and not need extra RAM. But if you're like me, I have 9 tabs with Firefox plus Windows Live Mail plus 7 tabs with IE plus a Spider Solitaire game, and if I only had 4 GB RAM, I'd be paging. Paging happens when all the tasks you have started cause a demand for more RAM than you physically have, so Windows will start swapping RAM in and out to/from a hard drive. Now comes the rub, hard drives operate in milliseconds, RAM operates in nanoseconds; that's a difference of a MILLION. Obviously that can slow you waaaayyy down if it's done a lot. That's why I have 8 GB. YMMV.

My guess is that 4 GB 1600 + 2 GB 1333 will be slower than 4 GB 1600 only. But just try and see for yourself.

Kees

because the bus speed will be adjusted to the slowest one. On older motherboards one had to either set manually, or put the slowest stick in the DIMM1 since that was the one which took a reading from the SPD on the stick.