Get one as soon as you can- they are fantastic.
I'm brand new to macs having ordered mine a few weeks back (then after, I read the rumours about the new updated ones...doh) Anyway, the 20" 2GHz with 2GB is a very quick machine, and to be honest, I don't think many users could complain, even if it is going to be updated next week.
Handbrake will encode to ffmpeg @ around 72fps (1500kps), so it's very fast for such a reasonably priced computer and best of all, it's near silent. I can't see there being too big a speed increase when the updates are released, and for who the iMac is aimed at, it's a fantastic product.
(If Hi-Def playback is a must, then just wang on a 23" ACD if you can afford it. Lovely jubly HD screen. The core duo will happily play back hi-def content

So no need to get the 23" iMac which will probably cost quid a few quid...even though a 23" iMac would look pretty hardcore

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