Here is what the spec sheet shows Dell offers with the D505:
Intel® Pentium M® Processors up to 735 (1.70 Ghz, 2MB L2 cache);
Intel® Pentium M® Processors up to 1.60 Ghz (1MB L2 cache);
Intel® Celeron M® Processors up to 340 (1.50 Ghz, 512KB L2 cache)
http
/www.dell.com/downloads/us/products/latit/d505_spec.pdf
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...