Find min and max values across two columns?

Hello. I've recently begun to use PostgreSQL in earnest (working with  
data as opposed to just having clever applications tuck it away in  
there) and have hit a wall with something.

I'm trying to build a query that among other things, returns the  
minimum and maximum values contained in either of two columns. The  
problem is that min() and max() only take one column name as an  
argument. Is there some clever way that I can craft an expression to  
find the min/max across both columns? I have yet to delve into any  
pgsql so if it requires that, then please go easy on me. :)

Example:

id | columnA | columnB
1 | 4 | 2
2 | 3 | 4
2 | 5 | 1


I'd like to be able to discover that 1 is the smallest value in  
either columnA or columnB and 5 is the largest. I don't actually care  
what rows they are in, I just want the values.

Thanks for your time! Any help or pointers to relevant reading  
material on this would be greatly appreciated. (I have been using the  
excellent PostgreSQL docs and an equally good book titled Beginning  
Databases with PostgreSQL by Neil Matthew and Richard Stones so far.)

--
Amos