Third Baby Dilemma - stick with 2 or twist to 3?

We have 2 lovely children; Katharine who is nearly 4 and Noah who is almost 2. Both my husband and I feel that our family is not quite complete and there's someone else waiting to join us, so to speak. I'm incredibly broody and in my heart I would like to conceive in the next 6 weeks for a spring 2012 baby, but my head is unsure about the practicalities. I think I could handle three - just about! But, would it be better to wait until my oldest is in school full time and my youngest part - time, or should we just seize the day?!! I don't know if we would be putting undue pressure upon ourselves, given that things haven't been easy for us as a family in the last couple of years, what with losing parents, and almost losing our youngest at 3 months. Any thoughts from parents who've stretched to three kids, and those who have stuck with 2? Many thanks :)