Hm, in the States the price difference between the 17" SD and the 20" is $400, or 214GBP... seems like the UK is getting a slightly better deal on the 20" or a slighlty worse deal on the 17" SD.
Anyways, $400 is a pretty god chunk of change, in my book. For the price of a 20" iMac i could get a single CPU PM with all the same specs as the 20" iMac, except it has a 9600XT GPU and a decent quality 17" LCD. The iMac will last longer, though, as it has better upgradability, and I can either get a bigger or second display later, or keep the 17" around for the next machine, etc.
Now, don't get me wrong, I'm buying an iMac myself, I just think that the benifit you gain in trade for the non-upgradability is a LOW price. As soon as you can buy any PM w/ a LCD display for the price of an iMac, you've lost your main advantage, in my book anyways. The 17" iMacs are great deals, and compared to THEM the 20" is, but compared to the SP PM the 20" isn't such a good deal, imo.
Rob