Updating packages and the rpm command

[Scott Lawton]

I found an updated version of the sylpheed mail client on rpm
find.net. 
What would be the rpm command to upgrade the version I already have
installed ?
[the man page for rpm is !! 800 LINES !! of text] 

Or do I need to uninstall the current, then install the newer, because
the names [of the packages] are different ? 

Here is what Software Manager tells me I have installed: 
sylpheed-0.6.1-1mdk 

Here is what I would like to put in: 
sylpheed-0.7.7-1.athlon.rpm
	or
sylpheed-0.7.8-1mdk.i586.rpm

Should I go with the version compiled on, and designed specifically to
run on an Athlon processor, or should I go with the most current
version [for the i586] ? 
Is there a significant difference ? I figure there must be some, or
people wouldn't be compiling for the Athlon. 

-- 
Scott

Sylpheed 0.6.1