D830 really. CPU upgrade question.
What is the best CPU upgrade the i can put in my laptop without changing out the motherboard?
Your options are limited and may be hard to justify based on the age of the laptop, but if you can get a compatible CPU for relatively cheap it may be worth considering. Your laptop seems to use Socket M or Socket P depending on when it was manufactured, so you will need to verify the socket and see what CPU options you have going forward. You can verify this with a program like CPU-Z or do a teardown of the laptop to verify the color of your CPU socket and the number molded into the socket as well (mPGA 478 (Socket M) or pPGA 478 (Socket P)). All pink sockets are Socket M and Socket P is traditionally white, but there are other known color variants for Socket P. Ideally you want the laptop to use Socket P, since you have better processor options available then Socket M.
The major restriction with the upgrade on this laptop does not accept 1066MHz FSB processors, so you will need to find the 800MHz variant of the processor you are looking for. Anyway, the Dell service manual can be found here since iFixit doesn't have a manual to cover processor replacement.