If you worked for the United States Federal Government and were paying into FERS, the Federal Employees Retirement System, or CSRS (the old system) and only have 1.5 years of creditable service then you are not eligible for payments from EITHER. You have to have 5 years of creditable service just to be eligible. See: http://www.opm.gov/retire/pre/fers/eligibility.asp and look under the heading Deferred Retirement for FERS and http://www.opm.gov/retire/pre/csrs/eligibility.asp under the heading Optional for CSRS. I'm going to assume we aren't talking about CSRS though, because that was eliminated for all new employees hired after 1987.
Good news though! Unless you got a payout of your retirement contributions you can always go back and work for the Feds for another 3.5 years to qualify. With 5 years of service at age 62 you could very well get the $320 a month payment you mention.
Generally speaking, the FERS payment follows this formula:
(1% * (AVERAGE(your highest 3 yearly salaries)) * Years of Creditable Service)
So for example, if you become well paid GS-12 employee for 3.5 years in the Washington DC area (I'll assume step 1, 2, and 3) (Ref: http://www.opm.gov/oca/11tables/html/dcb.asp), then when you turn age 62 you would be eligible and receive . . .
(1% * ((74872 + 77368 + 79864)/3)) * 5 = $3868.40 per year
Now, your personal retirement situation may be entirely different, but I'd go call your previous HR department and validate whether or not you are currently eligible, because unless you didn't state a key fact in your post that changes the situation, I do not believe you currently are.