Dis/Advantages of buying mixed notebook RAM?

Hi everyone!

Just a quick intro: I have an IBM/Lenovo laptop with 1GB ram, and it is PC2-4200 CL4 NP DDR2 SDRAM SODIMM (http://www-604.ibm.com/webapp/wcs/ [...] d=33075342)

I want to upgrade to 2GB, and I can either buy another stick of the same, or get a "faster" stick of PC2-5300. I know that the PC2-5300 will drop down speed to match the PC2-4200, so considering that they cost the same for me to upgrade, which would be better? The PC2-4200 says it has a CAS latency of 4 clock cycles whereas the PC2-5300 has a latency of 5. Since it will be falling back to the PC2-4200 speeds, does this mean it may even be slower?

The 2 sticks are as follows:
1GB PC2-5300 CL5 NP DDR2 SDRAM SODIMM (http://www-604.ibm.com/webapp/wcs/ [...] d=33075342)
1GB PC2-4200 CL4 NP DDR2 SDRAM SODIMM (http://www-604.ibm.com/webapp/wcs/ [...] 8425087607)

The only other real reason I can see to buy the PC2-5300 stick is if I upgrade my laptop later and re-use the PC2-5300 stick in its full capacity.

Alternatively, I suppose I could buy a 2GB stick of the PC2-5300, but I don't know if it would be worth while as the current 1GB stick would then be obsolete?

FYI it is a Thinkpad/Lenovo T43 with the following specs: http://www-307.ibm.com/pc/support/ [...] MIGR-58315

More details on the specs here: http://www-307.ibm.com/pc/support/ [...] -58317#mem

The processor is a Pentium M 760 2GHz (133Mhz clock/bus speed according to CPU-Z and 532MHz rated FSB)
Thanks!

Edit: Does anyone know IF I did buy a 2GB stick of the PC2-5300, whether my laptop would be compatible with it (the specs say max 2gb but it sort of implies that it is max 2 X 1GB sticks) or if the laptop would even be able to run the faster speeds ? (or would I have to phone IBM to find out)