With the age and spacing of your kids, I think you'll find it to be great! Both of your girls will be at an age of wanting to be helpful and will love and adore the baby.
I had a 22 month old that I thought would be SO jealous when I had baby #4. But she wasn't at all. I gave her the role of being big sister and being my helper, and she did great. She loved her sister, never seemed jealous, and really surprised me. So, I do suggest giving your younger daughter (and older too) the role of helper. Give her something to make her feel important...and, of course, be sure to give them continued focus and attention.
For me, going from 2 to 3 kids was the hardest BUT my kids were way closer in age than yours (the next youngest was 19 months old when baby arrived), which makes a huge difference. Odds are you won't be chasing your 10 year old around trying to get her to follow you out of the store;-) Honestly, I think you'll do GREAT with the spacing you have. You aren't dealing with several babies at once. You are dealing with a 10 yr old and a preschooler - that's awesome. I can't imagine it'll be too crazy at all.
If you like to read books, "Siblings without Rivalry" is awesome. I think anyone with more than one child should read it!
Hope you're prego!