help re specs for new iMac please.

Always been a windows person but over the past few years with an iPod touch,iPhone 4 then the 4s and an ipad 3 iv fallen for Apple so as my desktop is well past its sell by date I thought i'd treat myself to a new iMac.
Mainly use the computer for betting on betfair sometimes using automatic bet placement programs,Poker, surfing the web a bit of fiddling with photos (which I plan to do more of) small amount of gaming and watching films/tv.
I think I prefer the 27 inch model but no idea what upgrades are worth me getting if any so just looking for some advice/help.

Thanks in advance

Paul

Popped into Apple storeand the guys in there said the same "base model and maybe fusion" so I think I will go with that. Was budgeting on spending £2k so I think Mum will be getting an ipad mini for xmas.
Thanks for taking the time to reply.

Paul