Save some more and wait for newer cards like GT300 and see what you can get...
And ya... IMO The performance difference won't be 50%.
It depends on the games you play and at what res.
Oh, i forgot to say, my monitor res is 1920x1080, but i play at.. 1680x1050
Why don't you play at your monitors native res.?
Anyway, I know that the 4800 is a great improvement from the 3870, but I am with the others who say to wait a while. More competition over time will most likely drive the prices down a bit across the board.