Originally posted by Platypus
Slot 1 motherboards are typically unable to recognise high-density memory, this gives the symptom you're getting, only recognised as half the capacity. I think most current DIMMs would be high-density.
As well as being double-sided, does the new memory have the same number of chips as the older one, or only half the number?
Far not all the slot 1 motherboards are having this problem - I have at work ASUS P2B-VT (VIA693A chipset, slot1) motherboard working fine even with 512Mb PC-133 DIMMS...
So, problem can be with that particular chipset...

But yes, most likely you have high-density RAM - that's why your motherboard can't recognize it correctly. I was having recently the same problem with customer's Shuttle slot A motherboard, but fortunately, I had enough different 256MB sticks to choose from... .

In your case I would recommend to update the BIOS to the latest possible - sometimes it can help...at least, it helped me with my ASUS P2B motherboard to see 256MB PC-133 DIMM...
But be careful...