Well, ATI just cut the 4870X2. The new 5700/5800 series seem to be well built cards, but I fear it may be a while before they come down in price, and the 4870 may not last that long.
I would go for it. Theres really no telling how long they're going to last, but no fear. You could always pick up one from ebay if you really plan on waiting, even if ATI does cut 'em.