Just to preface, I'm really not sure if this belongs here, or Retardoland. It's such a great topic for a first post. (Long-time lurker, though.)
Since 512 MB of RAM just wasn't cutting it anymore, and there was a sale on laptop memory, I upgrade the laptop I use for Simming to 1 GB last night. Or tried to.
Inserted the new card in the slot (and yes, it was the appropriate sort, right speed, etc.). Computer wouldn't start. Removed it. Computer wouldn't start. Put in the new card only, computer worked fine, but only at 512 MB.
Not only that, but it was crashing when we loaded the Sims (fatal error), and we determined it was either memory-related or buggy download-related as removing the Downloads folder (which is sitting at exactly 1 GB of content atm) fixed the error.
My husband poked and prodded at the laptop a bit more today and discovered the reason that the old memory card wasn't working was because when we were fiddling with it, the connections got orange juice on them from when I spilled OJ on the laptop a year and a half ago.
So yeah, when your RAM chip starts acting up... check for OJ before you do anything else.