Stuck on iMac 27" configuration

So my early 2008 iMac along with a massive 2gb ram is driving me nuts. It's slow, the screen brightness uniformity is buggered and asking it to more than one thing at a time is not worth bothering with. TBO I think Mountain Lion has not helped either.

The time has come to upgrade and while getting the 27" is a must, after that I just keeping looking at the options over and over again.

I almost certainly get the GPU upgrade as I'd like to do some gaming and for it to look as good as it can, and of course buy my own memory afterwards, Now but after that, I'm stuck.

I use Photoshop but only as a hobbiest, it's not constantly being used. I don't edit videos either beyond the capabilities of iMovie. So do I really need the 3.5GHz i7 upgrade at £190? I'm thinking not, but it'll be a while till I would buy a new Mac so would the i7 serve me better in the long run?

Now for storage. Hmmmm. I'm highly considering getting SSD only. I hear it's a bit faster then the Fusion drive and more reliable (potentially)? Plus it's silent. I have a mountain of HDD's for mass storage so don't really need the Fusion flexibility and leaving it to Mavericks to decided what gets stored.

Now what size? For basic stuff the 256GB seems fine, however I'd want to use Boot Camp for some gaming so would I benefit more from getting 512GB? I assume Boot Camp and Windows works fine on SSD? Also, while today 256GB is fine, come selling time, is it going to sound pretty small and useless?

The 12 months 0% finance is great, but I really cannot afford to upgrade everything (forgetting the Apple ram), so I'm basically looking at either the i7 with 256GB SSD OR i5 with 512GB SSD upgrade

Any helps guys? I realise that whatever I get, it'll be a massive upgrade to what I have now, but I just want the best all round configuration I can get for my money