Greywolf, it depends on what you want.
If you want to get a complete AR15 something inexpensive then you can get the OLY plinker or on of the other entry level AR15's as most of them use cast lowers.
DPMS also makes their entry level AR15 with a cast lower.
For general shooting, plinking and fun a cast lower will serve you well.
The cast lower won't be as strong at a forged lower, if you are not using your lower as a support for leveling buildings you should not have a problem with it.