SRQ wrote:I don't have and have never seen a 512mb stick that's single sided.
They do exist, I have one of them.
The problem with high capacity memory sticks is that the compatibility is, lets say, rather questionable. As a rule of thumb you can operate under the assumption that an Intel based system will have issues with unusually high capacity memory modules. The most well known instance of this is the low/high density module issue primarily found on DDR2 and 3 modules, where peculiarities of the Athlon64/phenom/FX Memory controller and memory addressing is used to offer effectively double the capacity normally available on those modules (those modules are of questionable quality however and many boards don't support them either). AMD has pretty much always been more flexible when it comes to things like this at the cost of considering standards more like a set guidelines, while Intel is rather strict about those standards.
While I have seen an intel system running with a 512MB (double sided) SD ram module (Celeron 1200, don't remember the chipset) that was the only Intel system I have seen to do so. From my experience those are somewhat rare. I had much better luck with Socket A (K7) boards and 512MB SD-Ram modules.
The board you have is a workstation board and those are generally much more strict with the memory they run with than consumer boards because of validation and support policies, so I am really not surprised about the lack of support for larger memory.