The current quad-core iMacs should be a huge step up from your G5 iMac. If you already had a Core iX-based iMac, it'd definitely be worth it to wait until Sandy Bridge, but in your case, I think you're safe to take the plunge now.
I have my current G5 for five years, and I want my next Mac to last five years as well. When the refresh happens, instead of getting the 27" i7 quad, I plan to buy the 21.5" base (hopefully with i3 550 or even i5) right away and an iPad 2nd gen next spring for the same money. Will the i3 550 able to give me five good years?