there is a such thing as 1 gig ram modules, but they are extremely expensive and you are unlikely to find any ddr modules at all. I dont believe micron (crucial is a subsidary) manufactures it anymore. Might have to try kingston or samsung. They are sold as 16Mx8x4 modules of 512M memory, as opposed to the standard 8Mx16x4.
There are also motherboards with 4 gig supported memory, but ddr usually peaks at 3, and to be honest you'll probably only ever need 2 unless you are running a server for a renderfarm or something of that nature. The memory system is often determined by the chipset the manufacturer is using, I believe that the 760MPX, currently the best supported athlon mp chipset, does indeed have a 4 gig addressing system, any other chipset using the amd 762 northbridge should support 4 gig.
If your looking, the msi kd7 is pretty popular, though it has fewer pci slots than most, and 2 of those are 64 bit pci slots. The kd7 64 bit slots will support 32 bit pci cards though. It also uses the pc2700 memory slots. The asus a7m266d is also popular, but seems to be quite buggy to me. it does support 4 gig of memory also though.