Not in your ddc but here's my take on it: my ds was 3 when the baby was born and had a really hard time. Lots of anger, tantrums, hitting, etc. Not that I could blame him when his world had been so radically changed, but despite my dh taking lots of leave and my reassurances, it was pretty awful. What made everything much better was when he started preschool.
Suddenly, he had something to do that was special, just for him, that the baby had no part of. He loved being around kids his own age instead of being stuck home with a mother who was always nursing "that baby." His whole attitude really changed.
And for me, it was great because I finally got to have some time alone with the baby, to just lay him on a blanket on the floor and dangle toys over his head to watch him bat at them, without him getting trampled by his brother. To nurse him without worrying about someone hitting me because I couldn't get down on the floor and do legos.
If I'd had my second when my 3yo was already in preschool, I think a lot of the problems wouldn't have been so bad. But the baby was born in February, so it was kind of a long haul. If you're thinking about sending your older boy to preschool, you're in the perfect position, as he'll have all of September to get used to it before the baby comes.