Inspiron 1564 thermal pad conductivity

Hi, 

I'm cleaning out a clogged heatsink on an Inspiron 1564 and noticed the GPU thermal pad that sits between the chip and heatsink is dried out and cracked. I would like to replace this before reassembly. 

Obviously pad thickness is paramount to ensure proper contact between component and heatsink. Both Southbridge and GPU pads are 0.7mm thick, with the Southbridge one being dark grey in colour and the ATI GPU one a murky brown. I have found a supplier with stock of replacement pads sold in strips. I don't mind buying a strip or two as I often work on servicing laptops. 

Replacement pads come with different thermal conductivity ratings ranging from 1.5W/mK to 17W/mK. 

The question is, what thermal conductivity rating pad do I need for each chip?

There's a huge variance in pricing so don't want to spend silly money on a pad that's going to be overkill. 

120x20mm strips are:

1. 3W/mK      £1.72
2. 7W/mK      £5.43
3. 11W/mK    £8.71
4. 14W/mK  £16.75    
5. 17W/mK  £70.33 

Is there a way of calculating it if recommended ratings are not known? Any other suggestions?