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Worked fine for me on a 2.8 GHz Celeron laptop with only 512 MB of RAM, running KDE4 plasma-netbook interface. Could run Firefox and even watch youtube videos (until a certain Flash upgrade hosed the box and the reinstall never ran Flash properly afterward).
Yes, desktop effects are turned off (the onboard RADEON 7000 GPU doesn't support them), nepomuck/akonadi are turned off (it's an HTPC, no need for strigi or mail storage), and it tends to only run 1 app at a time (Dragon). But KDE4 runs beautifully on it, and it's very usable. It was our main TV show/movie access for several years, starting with KDE3, then upgrading to KDE4.2, then 4.4, and I think it was running 4.6 by the end.