I believe that once the domain is expired & reregister it is true the pagerank dissapears. Also you wont get the benefit of having an "old" domain. I would recommend purchasing it befor eit expires.
My current domain:
Worldwide Trading Enterprises Ltd was registered less than a minuite after it expired.
When I do a whois.sc on it
http://whois.domaintools.com/worldwidetrading.co.uk the domain shows as being registered in 2006 even though it was registered in 1998 the first time (bummer!).