Sorry, I meant to say Dashboard on Tiger. As long as you disable Dashboard then memory usage should be similar - Dashboard is a real memory hog if you leave it on though, each widget open runs all the time even when Dashboard is hidden and uses up its own slice of memory. Apart from the addition of Dashboard and Spotlight and a few visual tidy-ups (I much prefer the Tiger menu bar, for instance), I can't really see any major differences between Panther and Tiger, certainly nothing else added that could bog down an older Mac. Tiger seems a sensible choice though as it has a lot more support these days, and shouldn't die too quickly as Leopard is so bloated that there's going to be a lot of Tiger users left out there who can't upgrade. I've seen Tiger on a clamshell with 320 MB and it seems fine. Have you tried cleaning the favicons? (it was mentioned here once before as a way of speeding up Safari on an old OS X installation).
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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)