There are several site:
http://itknowledgeexchange.techtarget.com/server-virtualization/vmware-esx-35-upgrade-path-where-to-...
http://vmetc.com/2008/01/31/upgrade-to-esx-35-without-leaving-your-desk/
In any case the upgrade 3.0.2 to 3.5 is very simple: a fast solution is use CD, a better solution (is possible is to simply reinstall the ESX).
You could have some little problem with VC, if you're using MSDE ad DB.
Andre
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Andrew | http://about.me/amauro | http://vinfrastructure.it/ | @Andrea_Mauro