What software do i use to test my gpu's stability after OCing?

Since your going OC anyway get MSI Afterburner, that will OC easily, just remember to get into MSIAfterburner.cfg in its Program Files folder and change Unofficialoverclocking = 0 to 1 and your good to go. Then MSI AFterburner should come with Kombuster, just use kombuster if you see Artifacts or on Kombuster it should be squares, your OC screwed up. Or you can try Furmark (Bench and stability only) and that would show you artifacts on a messed up OC in yellow squares.

+1 for kombustor, there is also OCCT.
Btw, why you saaying to change unofficial overclocking from 0 to 1 ?

-.- are you cereal? New ATI 5XXX series cards can only OC by changing that setting.