I just switched to clipless pedals and without getting either fancy pedals or shoes it cost me a little over $200 so that will give you an idea of what to expect if you go this route. You could probably get by with $150 if you tried hard enough but something like PowerGrips would give you most of the benefit at much less cost so I would suggest that if you want to attach yourself to your pedals for now.

Trek and Giant certainly are good brands so I don't think that limiting yourself to those two is a major handicap. You should be able to find something suitable from them and if as someone else suggested your local shops can also get in Surly or Fuji there are two more good brands. I ride a Fuji myself and am quite happy with it. I would buy from the LBS if at all possible. They are a great resource for newbies and experts alike so establishing a relationship with one is a good idea unless all the LBS choices just suck as businesses. Assuming that is not true I think it would also be worthwhile to discuss the upgrade options on your current bike that have been suggested here with your LBS's to see what they could do for you price wise. They might have some ideas that would be attractive to you.

Ken