Hilary, I would suggest you consider 3 years apart, my only 2 children are 14 yrs apart as Things don't always go as planned... Needless to say I could get PG but kept losing my babies thus the 14 yrs between them.
I suggested 3 yrs. for you because at 3 most children are potty trained, can entertain/play/occupy themselves and have become more independent thus allowing you more time for an infant and any other domestic duties you need to take care of. In addition having a 3 yr old and infant you will be able to get out of the house if needed or desired without much of a delay.
If you have the children 2 yrs apart, it could prove to be overwelming
and you may not be able to enjoy the freedom to pick up & go. Keep in mind you may get some regression if you potty train once the baby arrives...
I always wanted my children 3 yrs apart, it didn't happen that way but o can now enjoy time spent with both of my children,my 2 "only's" :)
My sister & I were 21 months apart and we fought and there was jealousy
That I didn't even realize till we were in high school, for some reason it's still there today.
Good luck in your decision, just remember things
don't always happen the way we plan it may take longer than expected
just don't get caught up in being so specific have a buffer zone for those times when life just happpens :)