yes we don't know what will happen to the upcoming card. but at least i think nvidia know what happen to ATI's 2900XT so maybe they will do something to prevent that from happening to their new line-up.
Don't sell you're 4870X2 to buy a 5870. The 4870 X2 is 200$, the 5870 is 400$. You would waste 200$ and have even lower preformace than before. If you're realy dead set on selling you're 4870 X2, i might want to buy it to trifire with my EAH4870
Also, whats up with you're lag in crysis? You shoudn't be lagging. Im running a a 4870 1 gb, A 2.4ghz core 2 E6600, and 8gb ddr2, at maxed out settings, 4x aa, 1680x1050, at 30fps on vista. You shouldn't be lagging with a 4870x2. It sounds like you have some wierd bottleneck. Are you using a 1.8ghz 2001 pentuim 4 cpu or some other realy crappy cpu?