I have seen videos showing that the Core 2 Duo T6600 in a Dell 1545 can be easily upgraded to a more powerful CPU. I have taken these laptops apart before to replace USB and Power assembly, so feel confident I can do it.
There are plenty of CPU's on Ebay at decent prices so seems like a good option to get a bit more life out of the laptop.
What I want to know is whether there are any rules when doing this? Do FSB and TDP of CPU's have to match my existing one?
This is another socket P laptop and they are usually maxed out at 800MHz FSB. The laptop will fail to boot with a 1066MHz CPU if it is not compatible. When upgrading you should adhere to 800MHz, the best in performance/value being the T9300.
Inserting a 1066MHz CPU would be rejected by the BIOS and will fail to boot. Socket P laptops usually only accepts 800MHz FSB processors, unless you apply the BSEL mod on a 1066MHz processor. Applying the BSEL mod will allow you to boot with the 1066MHz CPU, however it would be downclocked and provide poorer performance over the T9300/T9500. Applying the BSEL mod to downclock the 1066MHz to 800MHz requires physical modification to the laptop, albeit it a basic procedure. For the BSEL mod on socket P, I'd look at this thread.
However with all that said, perhaps your laptop can accept 1066MHz processors and is not limited to 800MHz by default? I'm willing to research into this for you if you please reply back with your laptop's chipset. You can find out your chipset by downloading "CPU-Z" and looking under the mainboard tab, which states your Northbridge; if you could reply with a screen shot I'll look into your laptop further.
UPDATE: Your chipset, the GM45 natively supports 1066MHz processors. Hence you are not limited to 800MHz, you're free to install any 1066MHz CPU without any issues. However quad-core processors maybe problematic if your BIOS cannot read all 4-cores. Regardless any dual-cores processor on socket P is compatible.