If it were me, id wait and see what the 285 is like, should be a cracking clocker on a 55nm process, and its also a single card, not long to wait for it now as well.
if they'd got the core working well at 55nm it would have been out for 3/4 months already. The fact its coming out so late, and even so late they've barely boosted the clocks and all rumours pointing to a very very very small power saving over 65nm I wouldn't be expecting anything fantastic in terms of overclocking myself over a current 65nm version.
As said the 295GTX will probably beat the 4870x2, but look for it to cost upwards of £550, quite probably more and be almost non available.
As for 512mb not being enough for full aa/af at 1680x1050, well, I've yet to see a game that can't use 16x aa/af at that res with 512mb ATi side and only maybe 1/2 games Nvidia have a problem with, and really no mainstream games. You'll gain next to nothing from upgrading to anything at this point Crossfire I would say is only barely good at anything below 1920x1200.
Frankly the 55nm Nvidia parts aren't going to be setting the world alight and considering they'll likely want to shift 65nm parts I would say 55nm won't represent any value whatsoever, just buy a 260/280 65nm when they get prices slashed to shift them out of stock.
Thats if you have to buy, and, you don't.
Upgrading or buying at this point before the 40nm stuff is out is pretty much a complete waste unless you can find a complete steal on something due to being EOL.
The 40nm Nvidia part, well, getting to 55nm was a hassel, we'll have to see how quickly they can get their 40nm part working and what upgrades they make to that core. Theoretically to cut costs and raise profits again they need to follow ATi with a small core, but I doubt we'll see such a shift so quickly. So bigger core, more SP's/TMU's and higher clock. AMD we're looking at a 25% shader boost, 20-25% TMU boost aswell as a 30% clock boost, might be 40-50% faster than the 4870 we have now.