go for the 27" iMac. it'll suit your needs perfect. I've been doing HD video editing for 2 years on mine, audio as well; it sings. Photography you can do it any anything, unless you're using humungous file-size.
Do note that it comes w/o any useful customer installable parts except RAM, sans a superdrive, requiring additional HW, and can't run anything made for power PCs. If any of those limit your usage, then check around for a refurb or used iMac.