As I said, Spotlight and Tiger can both be completely disabled so that you benefit from the code optimisation without the extra overhead of the new features eg Dashboard can be prevented from loading completely so that there is no memory usage. I disabled Dashboard but kept Spotlight on my G3 and it works great (Spotlight doesn't seem to put much of a strain on once the initial indexing is done so I left it enabled - I even disabled the search indexing on my new C2D machine with Vista though, despite it being a very powerful system, as I so rarely use search in Windows and just don't need the indexing at all).
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Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)