if it is pci-e then ati hd 4650,4670,5670,5750,5770
if it is agp then ati hd 4650
i am picking Ati since power usage is low vs performance
also depends on quality of PSU.
if it is good name brand (Seasonic,Corsair,Antec etc) then running a card
rated a little higher (400w) should be fine if you dont have multiple HDs,optical,pci cards etc
this is a link to PSU calculator: http://extreme.outervision.com/psucalculatorlite.jsp
since it is a 7600gt it probably uses 4 pin molex for additional power (I have a 7600gs and it does)
you would have to check your power supply to see what connectors you have or dont have for external power to card.
a 7600gt draws around 50-60 watts if I remember right.
I would look for cards do not need external power connection (4650,5670?)
posting make and model of PSU would help also AGP or PCI-E