The way that Slush's pool pays for shares will play against you. Each share you solve (and you will solve very few shares with 340 kh/s) will decay very quickly. At slush's pool, the shares towards the end of the block are worth more.

I would suggest going to a PPS pool if you insist on mining. Don't get me wrong, it's great that you love Bitcoin, and want some for yourself, but at 340kh/s, you are putting a strain on your system and the gains aren't worth that strain.