Does your $140 budget include the PSU?
If not, look into a 4870 1Gb
for $125 after MIR.
If you need a PSU for that price also, I would recommend saving a little more and getting the above 4870 1gb and a Corsair, Seasonic, PCP&C, Enermax or Antec Earthwatts 450w+ PSU.
If you are stuck with $140 regardless, switch the GPU to a 4870 512Mb or 4850 as required by the cost of the PSU.