D.W.
Sure being full can help sleep but you also have to teach her how to sleep. Babies don't automatically know how. I highly, highly suggest Dr.Sears Baby Sleep Book, The Baby Whisperer and The No-Cry Sleep Solution. The more you know and understand about infant sleep the better you will be able to come up with a sleep plan and teach her *how* to sleep.
If she is taking at least 2 naps a day she is doing ok in that respect.
You don't say if your formula or breastfeed. One of my breastfed babies was a champ sleeper after he was taught how to sleep. The other one wasn't a good sleeper till she weaned.