Which 27' iMac for my needs?

Thanks again for the replies.

Tom_N- Thanks for catching my error- I meant Lion, not Snow Leopard.

webfrasse- You suggested spending the extra $200.00 upgrading to i7 CPU. What can I expect in terms of performance (especially while photo editing) with this and my business is small. Mainly family beach portraits, some pix for the magazines/newspapers now and then, plus my art.

As mentioned, I'm not computer savvy and the description from Apple goes right over my head: "Hyper-Threading (Intel Core i7 only) — a technology that allows two threads to run simultaneously on each core. So a quad-core iMac has eight virtual cores, all of which are recognized by Mac OS X. This enables the processor to deliver faster performance by spreading tasks more evenly across a greater number of cores."

If I install more memory from another supplier will I just have to install it and its ready to go or will I need to make adjustments? How would this affect the apple warranty?