Personally Xbox Live support doesn't affect my purchasing decision. Only in a few cases... like I don't think I'd get Unreal Championship if it didn't have Live play. Same with maybe Counter Strike. But a game like Halo 2, Amped 2, or many others (that will have Live support), I'd still get even if there was NO live.
"In my time we didn’t depend on high-tech gadgets like you do. We didn’t need a mechanical washing unit to wash our clothes. We just used a washing machine!" ~ Philip J. Fry