I have a Sony Vaio FX802 with 384 MB ram, split 1 x 256MB and 1 x 128MB across 2 banks. Last week I noticed that I was running on 256MB only so I re-seated the 128MB module and that seemed ok for a time, but again dropped out with a BSOD. I have since inserted a new 256MB module replacing the 128MB one, but this is still not recognised, yet it seems to be seated ok. Any thoughts?

I think, there may be some problem with the slots.
Try installing each RAM separately. If your PC recognize each RAM separately, then check Installing in separate slots.
The main motive of this excercise is to find whether RAMs & Slots are OK.
All the Best.

When adding new memory always take all memory out, reinsert the new 256mb stick into the first memory socket dimm1 normally, boot the laptop up, if the laptop boots then fine the memory module is ok, shut down and insert the original 256mb into the second slot, dimm2 then boot the laptop it should now boot normally, if not there maybe problems with the memory slot itself, give it blow to get rid of any dust and try again, if this dosent work post back.