Help for newbie please - which Mac?

Hi everyone,
I am looking to become a switcher but having been waiting for the mac mini refresh since January I have had too much time to research and now am not sure if I know what to buy!
I guess the best thing to ask is Mac mini or iMac considering what I hope to achieve?
The old laptop is going in the bin as iTunes is starting to seriously affect it's (and my) well being S L O W!
I have an ipad2 ATV2 and iPhone and ultimately want them to play nice and to continue to build my movie media library.

I really want to use PLEX instead of the plain apple tv interface on my living room HDTV and then move the ATV to the bedroom. So I was planning to hook up a mac mini via hdmi, However, I would need a keyboard and a mouse and would also look at increasing the hdd to 500gb and the ram to 4gb. Which now makes me think I should buy a basic iMac and hook that up to the tv.
That way I get a better processor and a nice screen for computer use at not much extra cost.

BUT, would the iMac be alright if it is left on all the time?
Would I get a good picture on a 46" HDTV via Dvi to hdmi without having to adjust anything?
If the iMac was in an adjacent room and I ran a hdmi through the wall to the living room could I activate the iMac from my iPad to start watching movies?
If I only want to use the Mac for HTPC, basic video or photo editing (beginner level stuff) and ripping and converting movies, is it worth the extra bother of an iMac and would a mac mini sat under the telly more than likely meet my needs?

I would love to hear views from anyone who uses an iMac as a HTPC connected to a tv

Sorry for the long first post