You likely blew the backlight fuse when it shorted out from a liquid spill - this is common. The fuse can be replaced and the backlight should come back on. The fuse is on the underside of the logic board. You can find all the info you need here: Detailed Logic Board Question. No Backlight..
I too had this problem (twice) with client computers. In both cases (for me), replacing the backlight cable that connects from the logic board to the inverter, fixed the issue. It's also worth noting that you don't have to completely remove the optical drive, display, etc as the iFixIt guides show. You can simply pop the top cover off and remove the LCD bezel and go from there, saving lots of time.
I too just replaced a failing 7300GT 128MB with a 7600GT 256MB card. I found upon disassembling the iMac that the heatsink mounting screws had come loose, causing the card to overheat and fail. An interesting side note is that the iMac used to have a faint screen burn-in covering the lower half of the display. After replacing the graphics card, the burn-in is gone! :)