You can install at least 4Gb of memory on most motherboards today, but apparently you shouldn't install more than 3Gb if you're not running a 64-bit operating system.
http://www.dansdata.com/askdan00015.htm
ZWarrior:
That would be correct, thank you very much MS. Of course that is only correct if you aren't running an Enterprise version of Windows.
Mr_Anderson:
You could thank Micro$oft for this too: http://www.theinquirer.net/?article=41577