Replacing gpu fan



If it uses the G92 style mount then almost Anything will fit that card and there are universal coolers that will mount on almost anything except for a few agp cards.

http://www.tomshardware.com/reviews/af6850-1024d5s1-ngt440-1gqi-f1-n450gts-m2d1gd5,2949-2.html

Any thing equal to or better than a Zalman VF900 will work on this card but will be dual slot.
 
@ Step last: You say 'space constrained' just to clarify, is there enough space to install a taller cooler? All of the aftermarket coolers will require at least two slots of space.
As an alternative: First remove and clean the existing cooler and replace the thermal compound with a premium grade material.
What case is it installed in? Perhaps you'll get improved results by adding a front or side mounted intake fan to improve airflow to the card cooler. A good test to see if limited airflow is a/the problem is to run the system with the side panel off and see how the temperatures respond.