spathotan :
Two 4850's do not cost the same as a single 4870.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102770 - cheapest 4850, x2 = $320
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121276 - cheapest 4870 = $250
Then there are the "hidden" cost, of needing a Crossfire mobo ($120-$200, depending on if you want 8/8 on a P45 or 16/16 on a X38/48. Or $180 for a 790FX w/SB750 for 16/16, or a $110 790GX with 8/8), and a larger PSU than you would use for a single card solution. And the 4870 is priced well for what it offers, and you arent really paying a premium for GDDR5, considering its $50 cheaper than the GTX 260 216 that uses GDDR3 and performs about the same. GDDR5 is better, period.
Dual 4850 will far outperform single 4870. They're not in the same league. The extra $70 is a lower % price increase for higher % performance increase.
As for ddr5 and ddr3, it's not nearly as important as most people make it out to be. To say that "ddr5 is faster than ddr3" is a gross oversimplification, and often wrong.
Use this formula for memory access bandwidth/capability:
(ddr# - 1) x single channel clock = effective clock
(Bus bandwidth / 8) x effective clock in ghz = overall capability
Let's do a bit of calculation:
4850 = 2 x 993mhz = 1986mhz -> 1.986ghz
4870 = 4 x 900mhz = 3600mhz -> 3.6ghz
gtx280 = 2 x 1107 = 2214mhz -> 2.214ghz
4850 = (256/8) x 1.986 = 62 gb/s
4870 = (256/8) x 3.600 = 115.2 gb/s
gtx280 = (512/8) x 2.214 = 141.7 gb/s
Imagine it as 2 rivers, one is narrower, but flows faster (ddr5, with 256bit bus), the other is wider, but slows slower (ddr3, with 512bit bus).
Of course, since 4850 also use 256bit bus, its memory is slower than 4870, unlike gtx280. But still, performance is determined mostly by the processor itself. Higher bandwidth, whether it's in the form of ddr5 or wider bus, is only a contributory factor. 4850's core is almost as strong as 4870, and benchmark performance reflects this, as 4870's performance edge isn't nearly as high as the 115.2/62 ratio would suggest.
Hope this sheds some light on ddr3-ddr5.