Sorry in advance - this is long - understand if it's ignored

So time to buy a new computer. At least i have decided on a Mac...
But reading the new imac thread here is completely beyond me and i'm after advice.
I basically know nothing about current specs, but like everyone here, i would prefer to spend money on gear to take photos not process them.
So i want to buy something to last a few years. i don't own a computer at present.
I have never bought a new computer and now the new imacs are out i'm hoping for a good deal on an old model or refurb or will spring for a brand new one if it is warranted.

It will be for photography mainly and i don't game or shoot video.
I want to view on a big screen...
I will eventually be using cs4 or 5 or LR3 - something.
But specs are beyond me..
People have told me save money, a mac mini or 13 macbookpro plugged into my HD TV will work for my needs..
I realise the color may be off on the HD TV..and it may be slowish - how slow i don't know..
(calibrate the 13 macbook pro screen and reference that when i need correct color to print)

So I'm leaning towards a 27imac..the new i5 quadcore.. big bucks
People (and the little voice inside my head) tell me get more Ram - okay 4 to eight will cost an extra $290 and it's a big improvement (i don't know at this size - is it ??)
Then the little voice says upgrade from i5 to i7 - another $290 thanks..(is that worth it ??)

Now the big voice inside my head is saying buy what you really need and spend the savings on other stuff. Problem is I don't really know what i need..or what will work well.
Is this just plain overkill to run post processing software. Is this just a case of new purchase fever...

I don't mind spending cash to future proof but I hate wasting cash even more..
I just want to buy and forget for a few years..

so whats really adequate...and whats a waste of cash