three ways.. well actually four, but I dont think you want the fourth..
one.. get a pci card like this one..
http://www.m-audio.com/products/en_us/Audiophile2496-main.html $99 and install.. you should be able to run your sony dat digitally into the soundcard and record (well, actually copy) the data to your hard drive in real time.. You will need the sony 7pin interface cable to do this.. If you dont have one, and the d8 is your only dat deck, you *will not* be able to copy your dats digitally..
two.. a usb interface like this..
http://www.m-audio.com/products/en_us/Transit-main.html $99 You still need the sony seven pin for the connection to your DAT, but this device runs into the usb port on your computer and you can record to your soundforge.. This is what I do, but with a product that is discontinued called "egosys u2a".. It works well for me for dat to HD transfers.. Im not sure if the product I listed is bit perfect or whether it resamples, but this choice is MUCH better than option 4..
three.. bite the bullet and buy a Microtrack 2496 (or similar CF based digital in recorder, but all others are more expensive than this unit). You will *STILL NEED* the sony 7pin, but you can bypass the computer transfer in a way. Just connect your DAT to the microtrack (7pin coax cable out > coax in of microtrack) and hit play and record (other details in there Im leaving out re: microtrack settings). Then you can just pop the flash card into the computer (or card reader) and drag/drop the files to your harddrive.
The fourth method is the analog transfer, which is the least desirable. Just play out of your d8 to the line in of your computer and record to soundforge. It'll work, but it is by no means a good way to transfer your dats to something HD based or burnable to cd/dvd... ::edit::also.. on the microtrack option.. m audio is discontinuing the microtrack 1 and will be offering a microtrack 2 sometime soon.. There is a thread in the recording sectioin for more info.. basically the same unit with some additions..
There are other PCI soundcards and USB interfaces available, as I only listed two, but searching this board for related material will give you all the info you need.. good luck!