if I install the new Garageband, will it overwrite the older version I have, and will my current projects play / load OK in the newer version?
The new version GarageBand 10.0.1 is only available, if you are running MacOS X 10.9.1 Mavericks. Have you already upgraded?
The GarageBand 10.0.1 installer will not uninstall GarageBand '11, but move it to a separate folder "GarageBand 6.0.5" in the "Applications" folder. You can use both versions with Mavericks. They do not interfere. When you open your current GarageBand '11 projects with the new version, GarageBand 10.0 will create an upgraded copy in the new format, but save the old version. The new version cannot be opened any longer in GarageBand '11.
For me, the new version is running without problems after the first bugfix has been released. But you should be aware, that GarageBand 10.0 is a 64-bit Mavericks version. Older 32-bit audio units, plug-ins, and device drivers will no longer work with this new version. Check, if the device drivers and plug-ins you are currently using will be compatible. Using incompatible drivers can cause crashes, see: GarageBand for Mac (v10.0): Unexpected behavior with Audio Units plug-ins
The new GarageBand is very differen from your current version. It has new features (drummers, arpeggiators), but other features, like Podcasts, are missing. Don't uninstall your current version, if you decide to upgrade.
Added: GarageBand 10.0 is free, but requires additional content as an in-app purchases, see:
As a previous owner of GarageBand '11 you can download this content for free, by using the command: "Restore Purchase" from the "GarageBAnd" menu.