Which memory, PC2 4200 or 5300 for T43

[Toastedling]

I have T43 type 2686, looking for a memory upgrade, should I go for PC2 4200 or 5300? Do they make any difference on a notebook? Also should I get two sticks kit? It looks like I already have dual channel enabled with one 256M and one 512M sticks.
Thanks in advance

[crazyfrog]

T43 will take both PC2 4200 and 5300, but 5300 in a T43 will just work as a PC2 4200. No difference.