My advice (purely from personal experience mind you) is to wait as long as you possibly can. I was told by Plunket to start Maya on solids at 16 weeks as her weight gain had started to drop off. So I did, and she loved it, but that was the start of all her probs ith allergies. If we had waited just a few weeks longer we MIGHT (not definite tho) have been able to avoid them. PLus we also discovered that her slow weight gain had nothing to do with being ready for solids and everything to do with the dairy she was getting in my breast milk.
Plunket family centres run a Solids talk for Mum's of 4-6 mth old babies, might be worth going along to that. I found it quite informative (apart from being told to start too early!) they told you what to start when etc.
Also, Angela (my boss here at OHbaby!) has just started her 5 mth old on solids, and wrote an article about it
here