Thanks for the replies. I was most interested in the one citing that
I must turn the memory speed down to 333MHz if three DIMMS are
installed. I wasn't aware of this. The 1GB works fine on it's own,
also it works fine with 1 other 512MB DIMM. The only thing that
doesn't work is all three DIMMS together! I have tried all three
locations for the 1GB DIMM and it makes no difference.

What do I loose by turning down thw memory speed, much? I just wanted
2GB of memory in my machine but didn't think it would prove so
challenging!

Thanks again for the responses.