performance counter alert on win2008

Hi all. I am trying to use performance counter alert on win2008 to generate
alerts when certain threshold are reached. I am monitoring processor% and
available memory. When creating the performance counter alert, i was prompt
to set the particular limit(in a textbox) before a alert is triggered. Hence
for available memory counter what is the limit i should put, if i would like
to be notified when i have only 1GB ram left. Should i set 1000 as the limit.
I am unsure as there is no MB or GB parameter. What about processor%? SHould
i put it in percentage terms? Pls advise. THks in advance.