I've come across an Elgato USB Stick that enables me to watch digital cable TV with my oldish macbook, which struggles a bit when doing so. Looking into buying an Imac now, size doesn't really matter (the ladies say), but nevertheless I'm not sure which specs to go for.

I want this thing to easily run HD TV, plus do a little internet/emailing/squeezeboxserver alongside too. Most of the channels here aren't HD yet, which also makes me wonder if watching SD on a 27'' imac is a wise thing to do at all.

So, it's 21'' vs 27'', and all the upgrade options:
i3 vs i5, standard 512MB Vid vs 1Gb, standard 4Gb ram vs 8 Gb and so on.
I'm confused. And unwilling to tick all the boxes.

I'm pretty sure any of the specs will do what you want. Are you sure the stick does high-def though? I didn't think USB had the bandwidth for high-def TV.
Also not sure why your macbook struggles, it shouldn't do really, TV isn't that intensive an application.

Stick definitely does HD.
Macbook is an older white Core Duo with only 64Mb shared video memory. I guess that's what's limiting its performance.

Ah yes, had forgotten about that. Well as the mini does high def, I'm sure the iMacs will do it with ease, so get whatever takes your fancy I suppose.
Seems a bit poor that the base level 27" model has the same gfx hardware as the faster 21.5.

Seems a bit poor that the base level 27" model has the same gfx hardware as the faster 21.5.

Exactly. And it has to drive a lot more pixel too.
So, I guess I'll go for a high spec 21'' to be on the safe side.