When thinking of DMC4 you need to be thinking could this PC keep up with a 360 / ps3 standard.
I would say that its slightly too old, although the low graphic settings may be ok. Upgrading is very cheap now (for a whole system). Its not worth indivudally upgrading because that board is too old. Find yourself a cheap 775 board, whack in a low spec c2d and 8800gt and some cheap ram....got a nice system for most games.
The PS3 and 360 versions of the game will be a lot more optimized so comparing specs won't really work. As you said, a Core 2 Duo with an 8800GT and at least of 2GB of RAM would probably be great for these titles.