I'm in the same dilemma as OP...
I currently have an iMac G5, RevB which has served me well for 4.5 years... still going strong, but starting to choke up on a large iTunes library, large iPhoto collection, etc...
I'm definately getting a 27" iMac... question is whether to get the C2D or the Ci5.
I look at it like this... $300 for a better (more future resistant) processor, better graphics card, and bragging rights (kidding!).
I use the computer for iTunes hosting / streaming, amateur photography (DSLR, high res jpeg, haven't fooled around with RAW yet), very occasional DVD editing, DVD ripping / transcoding for iPhone / iPods, plus the usual word processing / web surfing / etc that we all do.
I know for now the C2D would be fine for my needs (beyond fine for my needs, really), but all technology eventually becomes old... is the extra $300 worth it for the likely longer service the Ci5 would provide? In either case I plan to get 4 additional gigs of ram from crucial or newegg at a later date.
And since someone will probably ask... no the money isn't a big issue... it's more the principle of the thing. From a value standpoint, is the Ci5 worth it?