1.6A & 1.8A wouldn't be very good candidates. Some have hit the 3Ghz mark, but you have to be lucky.
I'd go with the lower to mid 2Ghz range. Most of those should be able to hit 3Ghz and would be the cheaper alternative. 2.8Ghz chips would just be too expensive and they don't OC as well as the lower (higher multiplier) chips.
Check out the
www.overclockers.com CPU database & see what certain chips have been able to do.