For mail: If Sylpheed uses mbox format mail boxes you should, in theory,
just be able to:

cat sylpheed-inbox >> ~/evolution/mail/Local/Inbox/mbox

For saftey, I'd first create a new folder in Evolution, close evloution,
then cat your old inbox to your new one. 

Other option would be to set up IMAP on your linux box, have evolution
connect to the local imap server, subscribe to the old mailboses and
move messages around. 

Contacts are trickier. Don't know what eveloution supports for inport,
or what Sylpheed supports for exports. I personally keep all contact
info in my palm pilot and sync my evolution address book with it. 

-- 
Andrew S. Zbikowski | Wireless: 612.306.6055
	http://www.ringworld.org
  "Life is what happens to you when you're 
   busy making other plans" -- John Lennon