You won't regret it. Sapphire, at least in my opinion, are the best ATi partner to buy graphics boards from. The quality of both the product and the customer service is always great.
I think we'll only see a few price drops between now and Q4 2006, then we may see the next gen cards which will cause this gen to drop in price dramatically.
As far as I am aware, the G80 was meant to come out this summer but has been delayed due to not being taped out. I have not heard anything about R500 coming out any time before the end of this year, so it could be then to catch Christmas and be early for Vista which has been delayed until Q1 2007.
Nope, because people like me who always go for the second-best or last-gen hardware will always want to buy your stuff cheaper than retail prices. Don't ask me though, because I plan to go DirectX 10 when it's available (and I just ordered an X1800XT if it makes you feel a bit better about timing.)
Were I in your position, I would've kept the 7800GTX. If you get enough in return for selling your 7800GTX though, the X1900XT will be worth it.