A 7600GS will be a negligable upgrade.
A 7600GT will provide 20~40% increase in performance if not more. A 7600GS will only provide 5~10% increase in performance, if not no performance increase at all.
So this is my suggestion:
EVGA 7600GT KO $132 AR
BFG 7600GT $113 AR
Plus 3rd party cooler like NV silencer 6 rev2 or Zalman vf700
EVGA 7600GT $109 AR
Plus 3rd party cooler like NV silencer 6 rev2 or Zalman vf700
Unless youve bought the card already. Stock NV cooler on the GTs have a bad reputation for being loud. Try to get rid of it by buying a 3rd party cooler like NV silencer rev2, Zalman vf700/900 etc