If you're shooting a rebel right now, I'd pick up either a Canon 550ex or a Sigma Super. I think there are used deals going on right now in the buy&sell section of this forum. Why? Because it will meet all your needs and they will be available at a price where IF you made a mistake and desire something else, you would not lose much (think of it as a 'rental fee'). Both flashes are available for around $200 (give or take $30). If you resold, you wouldn't have to worry about losing more than probably $10-15 plus shipping if you kept them in good condition.
Important thing is to get that flash and start that learning curve. Either will provide you with more than enough functions/capabilities. If you get a 420 or something with less functionality, you may regret it. I can't see you going wrong with either. In fact, I may end up picking up one of those myself

(and I already have 3 flashes)
Hope this helps.