There are a couple factors that you need to keep in mind. If you're utilizing more credit, and your total balances are raised significantly, then yes, your credit score might be affected.
The bigger question is whether you'll be able to afford the house payment and all the expenses that go along with it in addition to having the monthly ring payment. If you'll be squeezed, then you should do one thing at a time. I'd suggest getting engaged, paying cash for the ring, then shop for a home with your fiancee together -- the two of you will probably pick out something different than if you were shopping for a home solo. Plus, you can also pool your resources together at that time so you have a bigger down payment.