System only reads 256MB RAM when 512MB installed

[Snugglebear]

Pull one stick out, leave the other in the first DIMM slot. If the bios detects 128MB of memory, 99.9% you have high density RAM. Older boards like the BP6 cannot fully use HD memory, so you'll need to find low density stuff if you want all 512MB to be useful (essentially they're only wired to see half the chips, hence half the memory being recognized). A quick search on this board will provide you with all the details on how to recognize LD mem.