Does the card need a dedicated power supply cable plugged into it? if so make sure it is plugged in. What are the recommended psu specs for the card? keep an eye on the 12v rail requirements too, cheapy psu's stating they run at 450w might really only be capable of say 300-350 (not saying urs is cheapy, just in case) also as the card is OC it will need more power, so perhaps the psu isnt up to the job.
ALso try using driver cleaner pro to uninstall the old ati drivers properly, (always leaves files on the pc even tho u uninstall it) then install the new drivers. On a side note, when u get it working, for extra speed try the XG tweaked drivers for ur gfx card, they work great compared to standard nvidia ones, www.tweakforce.com
. hopefully ull be able to fix the problem!