UT & UT2004 (those are the only UA components that I have installed on this system) work just fine without the CD in the drive. iirc, there was some discussion about U2 having a 3rd-party no-cd crack applied for the UA release so I assume that it is OK. Don't know about Unreal but I would be suprised if it were any different from the other games.
That said, what I don't know is if the
installation will work well with a USB drive. You might be at some risk there.
I'm quite keen on getting an Aspire One also (actually, what I really, really want is HP's successor to the HP2133* but that isn't out yet) and as far as I can see, the current generation of 1.6GHz Atom-based systems are probably only up to running Unreal & Unreal Tournament satisfactorily (assuming that the very old games don't complain about wide screen modes).
Whilst the Aspire One might actually be able to run UT2004, it will either have to be in software rendering mode or with every conceivable texture and static mesh detail disabled. In any event, poor frame rates and looking rubbish.
*Yes, I know how poor the Chrome GPU is.