It's always best to use plenty of gas, I believe it only uses the gas necessary, so it's not like you lose money.
Thanks for the advice. Say, how can I see the required amount of gas needed to execute the contract (dapp)? Just asking, because I wouldn't want to send more Gas than the amount required.
The mist wallet makes an estimate, in geth I don't think it's possible. You don't need to worry about that, though. If you send 500k and the contract only needs 300k it refunds the extra amount.
Another thing to keep in mind is that if the transfer fails you'll lose all the gas used. If you're sending money to the contract I think it can only fail because of the lack of gas, or if the block gets discarded by the miner, but if you try to execute a contract function with the wrong value or without the right permission it will also fail, so in those cases it's best to use less gas (they typically don't require much gas anyway). The mist wallet protects you in some of those cases too, not letting you execute the function.
I'm gonna lock this thread and make a new one in the right section, since I can't just delete it... I have no idea why it got moved here in the first place.
