Maybe this is a well known fact for others, but I was unaware of this until I recently stumbled over it, so I thought it might be worth posting.
Long story short:
I have a older mainboard (Asus A8V deluxe) which needs PC3200 RAM.
Hopping through ebay, I found some quite cheap 1 GB sticks (seemingly meeting the required specs)
http://cgi.ebay.de/1GB-DDR-RAM-Speic...item3362a4a6d5
When the RAM arrived I noticed it wasn't working at all and wondered why.
Starting a thread over at techsupportforum (nice forum for computer tech stuff, btw) they pointed me to the high/low density thing which I hadn't heard of before.
http://reviews.ebay.com/Myth-Low-Den...00000001236178
I indeed got sold high density RAM without explicitly saying so on the sellers page (yeah, there's a cryptic hint which I now understand).