Original Generation 1 is really good, but OG2 is even better. The mechs are more homojapually delicious, the combat flows better, you're constantly upgrading to new mechs instead of going through most of the game with the same units, the story (convoluted though it may be) is fun, the visuals receive a nice little upgrade...
And yes, as has been said, the PS2 game is called Original Generations and is actually a remake of OG1 and 2, with totally redone graphics (3D maps, all the 2D battle art has been redone and looks gorgeous), a couple of new battle systems, new story twists, and more mecha.
Some of the new mechs from OG2:




