Go for a Zalman passive NB. Looks like it'll fit fine.
When I built my bro's PC I managed a cut down in half Zalman NB with a 7800GT on a DFI board (- even worse arguably) and temps have all been below 50C with a 120mm fan blowing from the front when a 2.6ghz OC and 260fsb (P180 case with HD cage removed).
If you have slightly more restricted airflow which might be likely but given you'd have to cut off less if perhaps no pins you should get pretty comparable temps.
Just make sure you aline the pins so the 120mm fan blows through the heatsink ie the lines of the same length pin are going horizontally. If I had some pics I could better explain but let me try and use a quick sketch...
- represents a shorter pin
= represents a longer pin
- - - - - - - - - - - -
- - - - - - - - - - - -
============ <---- AIR
============
============
============ <---- AIR
- - - - - - - - - - - -
- - - - - - - - - - - -
Pic of the Zalman if it helps
Alternative look to getting it replaced with that Evercool GPU heatsink or else the Vantec Icebreg 4 for a similiar but higher quality fan setup to the original.