I've seen it go both ways in terms of reimbursement or lump sum. The company I work for did a lump sum where they just sent me a check. The amount for relocation was based on how far you would be moving. I have friends at other companies who were reimbursed up to some maximum amount. Personally I like the lump sum much better as I can waste two days loading and unloading a truck as opposed to paying someone else to do it, and then pocket the majority of my relocation package. However, I was only moving ~200 miles.
In terms of looking for housing it didn't matter for me, but I do know of folks whose companies (Nike / Intel) flew them out for a week to look for a place to live.