Yes, you will be able to upgrade the cpu to a T7200. I also have the e1505 which was running a single core T13xx then upgraded to 1.83ghz 2m cache core duo t2400. Now I'm running the 2.00ghz 4mb cache core 2 duo T7200. A noticeable upgrade especially since I game on this laptop. Games which were not playable before are now very playable and benchmarks such as 3dmark05 have increased as well. I had also upgraded from a geforce 7300go to a radeon x1400 which contributed quite a bit to the performance of the laptop.
If you are comfortable using a screw driver, its very simple to do. http://ahwee.com/how-to-disassemble-laptop-insp6400http://www.insidemylaptop.com/disassemble-dell-inspiron-e1505-6400-laptop-removing-cooling-fan/
These links show you how to dissasemble your laptop. You will need to apply some thermal paste on your new T7200 cpu and clean the copper portion of the heatsink with isopropyl alcohol
Make sure you update to the lastest bios from dell. A17