I want to upgrade to the latest version of DD-WRT on my WRT54GL. I already have a previous version of DD-WRT installed. Can I simple install the standard version or do I need to install the mini version first, then the standard version?
Typically the Mini then Standard version path is the initial install away from factory firmware. You'll need to check the hardware version of your WRT and then find the appropriate download.
It depends on just how old your old version is, it's been a few years since they changed the upgrade process but that's a really old router so it may have been a few years since you upgraded. You need to spend a bit of time on the DD-WRT wiki and forums and make sure you upgrade it right, or just flash back to default FW and load the new version onto that.
Their wiki site doesn't specify whether or not you can upgrade from standard to standard, so I looked a little further. I found out that the mini version is used for the initial install because the default Linksys firmware cannot upgrade any firmware larger than 2MB. I should be just fine upgrading to the standard version and skipping the mini version.