I believe that depends more on motherboard support. For example, the MSI K8MM-V allows up to 2 GB of DDR RAM with only two slots. What kind of motherboard do you have?

K8T800 based Siemens Fujitsu motherboard that hosts 5 PCI and AGP, documentation on this is very sparse.

Another is Asus K8S-LA based on SiS760 northbridge and SiS964 southbridge.

The support is up to 2GB spread across 2 slots according to K8S-LA. Looks like this supports high density DDR SIMMs.

Thanks and cheers,

Just for my information.
Why does the support (or not) of some memory density is related to the motherboard ? Since the A64 have on chip memory controller.
Does the motherboard still has something to manage regarding memory ? (aside from physical connection ?)

Just because you have IMC, doesn't mean all boards use the same memory layout.
Some not connect ECC pins for example as it's extra feature.
More than 1GB per DIMM memory density for DDR1, usually requires ECC memory with doubled rank with Registred memory support being optimal. Not something run-of-the-mill MB has.