Hi Red,
First, which gigabyte board is it? DS3 is just a sort of a board 'class' Gigabyte uses in there model scheme. There are many DS3 boards.
If your board does support the 1333 MHz FSB, definitely go for the E6550 or E6750 which will be much faster than the 805.
If not, the E6400 is still a very good processor. Any of the C2D processors will be a big step up from any Pentium D 8xx series and will believe it or not consume a lot less power & produce a lot less heat. The 8xx series was notorious for high power consumption and heat production.