Hey Mark--
I generally do the upgrade first (if the old box is sufficient) and then move to keep the new box as clean as possible-- but that is just my personal preference. As far as words of wisdom, make sure you review the upgrade information as well as the information on moving to a new SQL server thoroughly before you begin. As the process is a bit different than a standard product upgrade. Also, make sure you consider the impact of the upgrade and move on other integrating products (FRx Report Designer, Business Portal, etc) that you use. Here are some links to help you out...
Information on upgrading...
https://mbs.microsoft.com/customersource/support/documentation/installationandsetupguides/gp10_upgrade.htm?printpage=false
Transferring to a new SQL Server...
https://mbs.microsoft.com/knowledgebase/KBDisplay.aspx?WTNTZSMNWUKNTMMYQLYTNSUKZPXKMUNVMKVZZXSTSMYWUNMYPOYXKYTZNUXNLSMK
Take care,
Christina