I partly want to say well if you can upgrade and it cost you barely anything, go for it. But honestly i think your system can handle almost everything extremely well, even if you upgraded to a i7 and things were a bit snappier, a few seconds faster when encoding things..is that worth it? Do you need those 3 seconds? I know i dont.
You're missing the point. There's no asking if it's worth it. He can sell / he sold his 775 setup and got enough money from the sale to buy a current setup w/o shelling out anymore money than what he made on the sale.
Most of us do this as a hobby and make every excuse to have the latest and greatest. For many of us, building a new system is something we enjoy.
If he had waited for six core processors to sell his 775 setup he wouldn't have gotten as much. With the iX setup he'll make more than he would have on the 775 setup when it comes time to sell and trade up to a six core setup.