The backgrounds in Grim Fandango and The Longest Journey aren't 3d, no, but the character models are. Those games look crappy when played without FSAA because the backgrounds are all pre-rendered and anti-aliased looking as a result, but the character models are otherwise horribly jaggy.
They do, in fact, both have a '3d accelerated' mode.
As to the FX5700...well those AREN'T the games *I* play (or other people play when they come over), those are the games my *wife* plays. Her computer, so her needs come first.
An FX5700 would be a great way to maybe meet her needs (FSAA those two games) and mine (fairly speedy DX8/9 rendering). *Maybe*. Need to know if it works in those two games, though.
It seems unlikely to me, as I understand nVidia has moved away from the super-sampling FSAA the GeForce4 Ti series did that worked properly in these games and started using a method similar to what ATI uses - that doesn't work.