My kids are almost exactly 2 1/2 years apart - April 15, '95 and Oct. 13, '97. It's worked out beautifully. My daugher was potty trained, and super excited about being a big sister - she did nothing but help with her brother when he was born - played with him, "helped" change diapers, held him, the whole bit. They're 14 & 11 and they are super close, and are only now starting to hit that stage where they annoy each other, and even that is pretty rare.
However, I should note that my daughter was exceptionally easy as a baby, and very advanced - spoke early, understood concepts well beyond her years, and was very self sufficient and entertained herself. No temper tantrums, no disagreeable behavior.
She did not prepare us for a "normal" child. My son was active, needed constant supervision as a toddler (seriously unprepared - we never had to child proof our house with our daughter - she just didn't get into stuff she wasn't supposed to get into), threw temper tantrums and was generally your typical kid. There was NO WAY we could have had another child within 2 years of our son. He wasn't nearly mature enough to understand it, and he had some serious Mama's Boy issues. He's awesome now, well behaved and very thoughtful and would have been a great big brother at 5 or 6, but not 2. Had the birthing order been reversed, there would have been at least a 4 year age difference in siblings.
So I'd say it depends more on the personality of your daughter. I personally suggest nothing less than 2 years - for diapers alone, if nothing else.
Good luck on your family planning, and congrats on your new (ish) addition!