Comming from a Dell 521 owner w/305w psu (backup system)
Yes it can, but for how long?
7800GT you would be ok with (20w less than 8800GT)
You can with the sata to molex adapter.
But for piece of mind why not spend $35 for a 30amp/80% efficient psu with the dedicated pcie x16?http://www.newegg.com/Product/Product.aspx?Item=N82E16817371006