How to save the game

I don't know about the best way to get studs, but the way my son and I did it was to go to Episode V, and to the level with the Millennium Falcon. You have to have the "Poo Money" unlock, and it really helps to have the unlock which makes your radius to collect studs much larger. There's a path that gets outside Hoth base (just a small area, and it doesn't lead anywhere). There are two Tauntauns there. Each player (or one player with a remote in each hand) can both get onto a Tauntaun. Arrange the Tauntauns so that their tails are touching, backed up next to each other. Then just keep hitting the button, and collect each other's studs.

I did this for a half hour while I watched TV (with the Wii in a picture-in-picture) until I had enough money to buy the next stud multiplier, and then did it again, and again.

The stud limit is 4 billion.

You are correct, there are no Live achievements available for the PC version, only the games listed underneath their catalog as LIVE enabled have achievements associated with them. You can also see a list of PC games with achievements at xbox360achievements.org which matches up with the Games for Windows catalog.