Obviously the one on top with both the faster Pentium M processor and larger cache will have more power.
So any of those should work. If you want to go larger, and I am not sure if you can, then you need to check with Dell - although it would not surprise me if they simply said that was the limit. But I would not go larger without their confirmation as their could be BIOS or power supply issues. Although you might check the Dell forums and see if anyone has found anything more powerful that works.
You might also check your manual about upgrading, I have not done it but have read that an upgrade like that on a laptop is often a lot more challenging than on a laptop. I found the service manual directions: