Synaptic Package Manager is merely a GUI for apt-get. So it may prove useful for user-friendliness.
I don't think it's necessary if you have GDebi. Just get used to searching the Ubuntu Packages list under distribution Hardy (or whichever base you are using), double-click the i386 file, and select a mirror to download from.
Provided you can then find the package to double-click on it, GDebi should sort out all your dependency issues for you.
Nevertheless, it also allows updates to be done via GUI, so you may want it…
sudo apt-get install synaptic
Another potential option (albeit with even more dependencies) is to use apturl
apturl allows a website to include links to install packages (with dependencies) directly on the surfing machine. You could then potentially borrow the OzOS apt:foo list to double-click applications to install.
To install apturl (and its dependencies):
sudo apt-get install apturl