The spouse of a legal permanent resident must wait for a visa to become available. You filing for her at this time grants her no right to remain in the US. The current wait for the spouse of a LPR is 5 to 6 years. During that time the spouse must maintain his/her own legal status. Your girlfriend cannot qualify for a change of status since she is currently out of status and would need to exit the US to obtain stamping for a new status. Once she exits, she may be prohibited from entering the US for as long as 10 years depending on her overstay. Since she is illegally present, she may be detained and deported if she remains.
Overstays are forgiven only for the spouse of a US citizen. Although you may apply for naturalization in Dec. 2010, it will likely be late 2011 (or even 2012)before you actually take the oath based on the processing delays.
Her best option is to leave immediately before she accumulated a 180 day illegal presence as she would not incur a bar. Overcoming a deportation would complicate her case. Violating US law has consequences; she must make a decision quickly.
Your girlfriend cannot remain legally in the US at this time.

