Welcome to the forums. I'm sorry for what you and your wife have suffered due to preeclampsia.
Here is a link giving some general odds of re-occurrence: viewtopic.php?f=19&t=331
Some of our posters have seen a MFM (Maternal Fetal Medicine specialist) pre-conception to test for underlying disorders and talk about how any subsequent pregnancies should be managed. Unfortunately we don't have a way to prevent preeclampsia yet, though treatment of some underlying medical conditions may help reduce risk in some individual cases.
The odds for subsequent pregnancies tend to be a little better, maybe because subsequent pregnancies can take advantage of some of the work done by a previous pregnancy in uterine spiral artery remodeling.
You could also talk to her docs about the possibility of taking low dose aspirin: http://www.preeclampsia.org/women-encou ... -providers
I hope this helps answers your questions somewhat. If not please keep the questions coming!