Currently we use Delphi 2007 because our application and some of our components are not compatible with Unicode. If and when we upgrade is it better to jump directly to Delphi 2010?
Propably, but I wonder if there is other compability issues except unicode?
How is performance, memory requirements and stability of those versions?
If you decide to upgrade, then yes, go with the latest version.
No doubts, you should go directly to D2010, not to D2009 - in this aspect D2010 is actually more like D2009 SP1 than a fully new version, just with some nice additions like an updated RTTI. D2010 has no new known compatibility issues, has better stability, etc.
Delphi 2010 is definitely better than 2009, by not much though.
Don't expect your upgrade to be smooth.
For example: Delphi has a new version almost every year. Then why it never helps you with the transfer of the settings between the versions? Why all components are written in a way to lock you to the latest known version to the component? (this is not necessarily problem with Emabracadero, but they could provide guidelines)
Check if your components have '2010 version or you might need to fix hundreds of small but problematic lines of code.