Regarding Buddhist ideas of Karma and Rebirth I will refer you to the recent post and comments at Progressive Buddhism.
Plenty of people far wiser than I have put in countless hours thinking about these matters. For my own part I will again agree with Kant (link to God part 1) that certain questions seek to go beyond the mind’s capacities. In a time when many thought they had ‘proved’ the existence of God, Kant deftly showed the fallacies of their arguments. Yet Kant knew that striving after such ultimate answers was and is still a fundamental aspect of being human. For our part all we can do is allow these questions to arise and to ensure that our own reasoning is clear. Greater clarity is the gift provided to us by generations past and the debt we owe to the next.
The Buddha, too, avoided what he saw as merely speculative metaphysics. If his words wouldn’t aid in the removal of suffering from a student, the Buddha would not teach. He taught simple doctrines of generosity and harmlessness to many lay-people. To only a few did he teach more complicated doctrines such as non-self (anatta) or impermanence (anicca). These were dedicated to the monks and nuns, those who had committed themselves to understanding the deeper truths of his teachings.
But those deeper truths were still only those that helped along the path to nirvana. It is said that the Buddha’s teachings are like a few precious leaves compared to the forest of his knowledge. The implication is that he could have taught all sorts of things, from horse-riding and archery to walking on water and through rocks, but these were seen as a waste of time compared to the vast project of eliminating one’s fetters (i.e. greed, hatred, ignorance) and achieving freedom or awakening.
~As I mention in a comment on the Progressive Buddhism post above, the ideas of karma and rebirth sometimes play a role in my life. They ease my worries or spur me to cultivate virtues. This is fine. So long as there is positive practical/ethical outcome from these beliefs, I see them as valuable. If ever I were to become obsessed with past-life issues or karma I would hope to be slapped back into the present life and moment as quickly as possible.
As for God, I still have not found one worth believing in (link to God part 2) . I know that many others have, including perhaps many Pure Land Buddhists. If belief in God brings benefits, let it be held, but if they are a source of anguish, it should be released.
What I have come to most in these posts and recent Buddhist studies is that it doesn't really matter whether one believes in God, karma, rebirth, etc. (I sense a collective ‘duh’ from wiser readers everywhere.) What matters is how we live our lives, how well we are able to translate our beliefs, whatever they may be, into the service of humankind. Mental labour should thus be spent far less on the cogency of belief, and more on the path to awakening.