A 9800GTX+ to SLI with shouldn't be hard to find on Ebay.
Beyond that I wouldn't say any card is 4 years worth of future proof. Just buy something that is adequate and a good deal for the money and repeat as necessary. Best value for the money right now in general is the HD6870 for $160 but with your card racism I'd direct you a tier higher for the GTX 560 Ti. I would get this while it is still on sale(ie today);http://www.newegg.com/Product/Product.aspx?Item=N82E16814127565