AmigaOS itself can address upto 4GB of memory. However, areas above 2GB can confuse applications (signed compare vs unsigned compare), and the OS itself (bit 31 has special meaning at places).
In practice the maximum memory safely addressable is 2GB. The maximum possible memory size can be calculated by substrating the first 16MB and the zorro areas needed for the system operation. The maximum possible memory size should be roughly 1900 MB.
PS. My Peg2 has 1GB of DDR400 memory. :-P
