Tyson glass prices have been coming down which in turn is driving the used market down too. I use to see the 7-14mm at around a grand but suddenly I have seen a few much cheaper used.
I got my 12-60mm for less than half of retail and was in mint shape. But I also saw a Nikon DX3 for $2500.00. Get one of those and you could be set for life. You can find some very good deals on craigslist if you just keep looking back.
If Olympus puts out a new pro body this year or early next year then the prices of the E3 will drop.
I think the D50 is basically an entry level camera much like the Oly 500. It is tough to compare the d50 to a 510.
But I am glad you are staying with Olympus. I think you need to shop hard for a 50-200mm.