I don't think I've read your previous threads so I don't know the intended use but I'd go for the £749 machine as a good compromise. 128mb of video memory should be fine for most things - even a degree of gaming.
If you are into gaming in a big way, spending an extra £40 might be to your advantage.
You've also got the extra 20GB hdd space which will make a difference.
I could also argue the other way round:
Buy the£673 machine and use the money left over to buy a good external hard drive.
As I don't know your needs or what hardware you already have, it's not an easy decision.
Having said all that, if money isn't a problem go with the £789 machine - it is good value for money.