About GT 280 being two times faster than 9800 GX2:
It will also depend on settings you run. With 1680x1050 4xAA one can't expect that much, but with higher settings yeah.
1680x1050 8xAA for example: 8800 GTX pulls 40% higher framerates than 9800 GX2. Actually 9800 GX2's framerates drops to 8800 GTS 512 level.
Basically with those HIGH settings 9800 GX2's memory bandwidht is just not enough and 8800 GTX/ULtra kind of cards will get ahead. Now add GTX 280 which is pretty darn good compared to GTX/ultra..and they take GTX/Ultra's best character (high mem bandwidht) even further
GTX 280 compared to 8800 GTX/Ultra:
-87,5% more SP-units [SP units on GT200 are also around 50% faster than those in G80 and G92]
-150% more TMUs
-33% more ROPs (well since 16 would be enough this doesn't matter)
-33% wider memory bus
-33% more vram
GTX 260 compared...:
-50% more SP-units
-100% more TMUs
-16.666..% more ROP's
-16.666..% wider memory bus
-16.666..% more vram