I believe 2 GB is the max, if you have a extra 2 GB stick you can try it with 3 GB and see if it boots.
Some of these earlier models could use 2 x 2 GB for 4 GB total even though the specs listed 2 GB as the max. of course if 4 GB was installed and worked, only around 3.25 GB would be accessable with the OS due to the chipset limitation. but Dual channel operation would be working as well.
Its on the 915 chipset so 2 GB is the (chipset) limit. I think that installing anymore than this will cause the machine to stop booting. The 3.25 GB limit that you don't have to worry about is the 32-bit Memory Mapped I/O limit. This limit is more of a combination of chipset and CPU limitations.
(There are also BIOS and OS limitations but these are not that important)