Well, first of all, if you have saved up 3k, then you should look also at the Mac Pro - in which case there would be no need to wait due to the sheer upgradability of the Mac Pro.
If you stick with the iMac (and there are many, many reasons to do so), then definitely wait till the Leopard release. Although there may not be any hardware upgrades with it's release, its nice to get the latest OS (and, perhaps iLife 07).
If I decide to go ahead with buying an iMac, I'm waiting till at least the end of summer. I want to know how well the iMac will run Vista, and I also want to see whether the graphics card in the top end iMac will be improved. Also, I have a feeling Apple will make a minor re-design to the iMac to allow for two hard drives. This seems logical as, given the emphasis Apple puts onto 'digital content' (movies, music, photos, etc) in all aspects of its business, and how increasingly common it is becoming in the mainstream to have a backup drive (external or internal), Apple might want to provide iMac users with an easy, integrated approach to backing up their stuff.