Closing credit accounts?

[TimM]

I currently have 3 credit cards and an MBNA loan open. They are:

GM MasterCard - $18,000 limit
Chase MasterCard - $18,500 limit
Discover Card - $5,200 limit
MBNA Credit Line - $18,300 limit

I only use the GM card and pay the balance each month within the grace period. The rest have gone mostly unused for the last few years. Is there any advantage or disadvantage to cancelling some of these other cards? Will it help or hurt my credit? Will it help or hurt getting a home loan, say, a couple of years from now? I'll probably want to keep two cards.