Looks like if you customize a new iMac then your ship goes from immediate to 4-6 weeks. I am debating on upgrading to the HD 4850 and wonder if it is worth it? I do game with my iMac with Windows booted Battlefield and want the speed.
For the new top end iMac the only way I'd go for it would be to add the $50 and order the Radeon HD4850. I don't care how long I'd be waiting. Either that or I'd be looking for a refurb 3.06 from the last revision. Refer to this thread.
And anyway, it's always worth waiting for something you ultimately won't be able to change later.