Teh reason your LG looks better on SD channels is because of its proprietary XD Engine that boosts low level signals. Samsungs are notorious for looking the worst of the bunch on SD; not to mention with Time Warner/Bright house cable, most channels in their "Digital Line-up" are in fact still standard. Only DirectTV has pure digital or HD programming. Try swapping your composite cables, for some true component cables. My guess would be a bit of an improvment overall towards line noise, etc...
And dont get a PS3 just to get a blu-ray player; A.) its only 1080i, and B) It wont up-scale normal DVDs. for $100 more, get the Sony BDSP1...