A lot depends on how far you are planning to upgrade - one level isn't
too bad ( though that can go wrong ) but more than one version upgrade
is just asking for trouble IMHO - i.e. 10.04 to 10.10 to 11.04 ( can't
go direct from 10.04 to 11.04 ). Only difference is that 10.04 is a
LTS version and could be upgraded to 12.04LTS in one step when it
arrives. Personally, given the huge changes that 12.04 is going to
have I'd think a clean install is the better way to go anyway.

You can use the Ubuntu install CD to do a manual install and erase the
old Ubuntu partitions / create new ones. Get the chance to set a /home
partition too so future clean upgrades are easier :-)