I have taken the plunge and updated to 10.5.6 Vanilla and now Safari 4 does not crash anymore, also nog with the "Top Sites". I don't know what is different, but apparently it likes 10.5.6 Vanilla more than 10.5.5 Voodoo.

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Apple Macbook Pro 15" 2.93GHz | Apple 24" LED Cinema Screen

iAtkos v5i 10.5.6 Vanilla

Machine: Acer 7730G
Memory: 4GB DDR2/667
Storage: 2x 250GB
Display: nVidia GeForce 9300M GS