Don't worry. Although Ati has a bigger GDDRam than nVIDIA but its bit interface is smaller.
The bandwith and the GPU of a VGA is the most important.
In this example, GTX285 is more powerful than 4890 :rockout:
in my experience, nVIDIA Card have a better fps than Ati. Especially in some Game support PhysX such as Crysis, FarCry...
So... that 80$ will help you more in games :pimp:
Games nowaday don't support the multi core GPU Card so I think the gtx285 is the best choice for you. Anyway, it have a high fps in all games that gtx295 and 4870x2 can't beat.