I really don't know anything about pearl necklaces... (the literal meaning, not the figuritive one... get your minds out of the gutter!!
)
However, I am also looking for a necklace for my wife. The rule of thumb for me seems to have always been this: You can't go wrong with diamonds.
I prefer a simple yellow gold chain with a single or 3 diamond pendant-thingy hanging from it. Locally where I live, I can get a 3 diamond 1/3rd carat total weight necklace for about $300, or a 1/2 carat total weight for 5-600 bucks.
It seems to me that with every woman I've ever dated, including my wife prior to our marriage, diamonds always work well. Be careful with it though.... too small and they may not appreciate it... too large and it could look gaudy or tacky.
The princess (square) cut diamonds seem to be pretty popular these days... although they seem to be a bit more expensive than the marquis cut because they have more facets (more sparkle, brilliance, etc.)
If you do shop for diamonds, make sure you read up A LOT or take someone with you who knows what they are doing to ensure that you don't get a raw deal.