I'm a bit confused about all the memory types and various bus frequencies of modern consumer PCs. Requesting expert help on the subject.

So far I'm confident that:

• I have an Asus X51L laptop with an unknown set of configuration options.
• The CPU in there supports PAE, so I still have a chance to extend the memory beyond 3GiB; and the upper limit of the system is 8GiB. (?)
• The laptop has two SODIMM slots, one of which is occupied by a 2GiB bank, and the other one is empty.
• dmidecode and lshw tools consistently state 533 Mhz frequency of the bank.

The last one confuses me the most. I failed to find out characteristics of the northbridge in this laptop, and still can't figure out what DDR2 to seek for. Is it DDR2-1066? Or, rather, PC2-8500/PC2-8600? Wouldn't a DDR2-800 bank harm the system's performance?

Which kind of modules should I look up in stores?

Update: I have bought a 2 GiB DDR2-800 SODIMM, and it seams that the system can't handle 4 GiB of memory. When installed by itself in either slot, both new and old bank (which btw happens to be marked GDDR2-677) work just perfectly; i.e. any configuration resulting in 2 GiB works. When both banks are installed though (totalling in 4 GiB), the memcheck86 tool produces horrible artifacts and crashes, and system reboots; an Ubuntu system can be started and even logged into a Unity session, but the system reboots too in this case from even a minor RAM load.

So it's pretty obvious to me now that this laptop doesn't support 4 GiB of RAM or more.