It turns to greed I think when someone has much more than enough but doesn't do anything with the extra.
But isn't this somewhat the same problem? If a poor person saves we applaud it, if a rich person saves he's "hoarding". What if the rich person just thinks they are a great investor and wants to give it all away when they die? Their motivations must not be greed. These behaviors are again things we find healthy in people who simply are not rich.
If the rich use the money, they waste it lavishly ("flashing the cash, gaudy, wasteful") if they don't use it they hoard it senselessly. I really don't think their activities here are the key factors, just the rich part. At some point it becomes conceptually too much, and no matter what they do with it people start to view it as being wrong, unless what they do with it is to immediately start rectifying the error of their ways (of having too much money) by getting rid of it.
But when someone has that much and isn't willing to "give back" in any way but the bare minimum (taxes) -- and further, tries to beat the bare minimum via creative accounting and offshore bank accounts -- that gets into greedy territory IMO.
Why is it ok for us not to be giving to those poorer than us then? There are people who are as poor in comparison to you as you are to the rich, but if you don't start giving your money away to them significantly (the rich give, so it's the amount that is in question) are you being greedy?
And if we, within the law, try to minimize our own tax burden is it greedy? At what point does the same activities we all engage in* suddenly become greedy?
*I am excluding cheating, tax evasion etc. Those are clearly wrong regardless of who does it.
I think greed is also a manner of doing things. As in, someone can reach a net worth of say 5 million dollars greedily or not. The greedy person would likely screw over a succession of people, cheat, lie, etc., all to maximize $$ without consideration of other elements.
This, to me, is the only definition I find valuable. Thing is, most people simply seem to assume that anyone rich got there that way. They will call them greedy on the sole basis of the discrepancy in wealth, without having any specific act of greed in mind.