they will all work on your mobo.
All depends what monitor you have and how much you are willing to spend tbh. At 1680x1050 I wouldn't go anything lower than 3850pro 512 or 9600GT 512Mb.
AFAIK 3850pro 512 < 9600GT < 3870XT < 8800GT 512 < 8800GTS 512.
As said I'd you'd be very happy with the GT especially at the price they're selling now. I wouldn't go for the overclocked GT version, money is better spent towards a GTS.
In terms of price/performance the list is a little different though.
More like exactly the opposite.
That said, I've just bought a 8800GTS 512mb....
