you would very likely get nearly identical performance with 2 8800gt in sli to that of a 9800gx2. 9800gx2 is 2 8800gts's pasted together with slightly lower clocks, basically sli that you can run on any rig. A slightly down-clocked 8800gts should perform like...yep, you guessed it, an 8800gt. You can spend $380 right now and get 2 of those msi OC 8800gt cards at newegg with the dual slot coolers right now, that is significantly less than you'll be able to get a 9800gx2 for. Plus, with the OC on those cards you'll probably get BETTER performance than 9800gx2.
rollo said $449 a couple of weeks ago, but nordichardware seems to think that it'll be $599. Basically, it depends on the performance. If it's $449 then it's that cheap b/c it's close in performance to 3870x2. I would be shocked if it wasn't at least 10-15% faster, so $500 + seems likely. If it IS released at $449, however, then expect 3870x2 to rapidly drop to below $400, so we the consumer will win either way!