My advice--buy a better main card, run both monitors off it, and sell the current card on ebay. Should keep you close to your desired price when all is said and done.
Really, though--I have a first gen G5 and run two monitors, both hi-res art monitors, on a Radeon 9600pro with only 64mb RAM (so 32mb per monitor). I have had zero problems ever, far less than the fx5200 with double the RAM in my G4 (yes, I know, much lower quality card, but still). Granted, I don't game, but I do do video editing, intensive graphics work, and often watch movies on the spare monitor when I don't need it (while working on the main one). RAM isn't everything, and if your graphics card is processing everything it needs to, you don't need tons of RAM. So you're probably going to be all right with what you've got, I'd say test it out first and see if you're not happy with it.