Adding Memory to Aspire 5601 AWLMi

I had problems on boot-up, the blue screen of death and then multiple re-boots. Called Acer and they said 90% of the time it's memory. So I took out the 512MB memory I'd added and didn't get the re-boot problem. Then re-installed the 512 MB additional stick and seems to be working. I checked the specs and found the 5601 comes with 533/667 MHz memory, and the stick I bought at Circuit City says "512MB PC2-4200 CL4 200-Pin SODIMM"

Does this mean it's 420 MHz? If so, shouldn't the factory memory run okay together with this add-on, just slower?

Alternatively, maybe the computer is running okay now because I re-seated the added stick a little better??

Any help would be appreciated. This is my first laptop. I like it, altho the partitioned hd is a bit of a bother.

 

PC4200 memory is 533mhz, which is fine. It was possibly due to a seating area in the module bay, if everything is working ok now then leave it be.

 

Much thanks sionyboy. The labeling of memory is confusing. I'm crossing my fingers that I don't see that blue screen any more.