The 4870's have dropped so much in price already, that it's a rather good deal for now. Take for example, a Sapphire 4870 1GB is only $140 on Newegg.com and cannot go much lower even after ATI releases 5870 cards.

However, 4890's have a lot more room to drop in price, so if you do not want to be disappointed, it's better to just wait 2 months or get a 4870 1GB now.

There goes my expert wallet advice!