PowerIcons, VisualPrefs and AmiStart have been mentioned, and while they are all great enhancements (except AmiStart - I hate start menus) they are not what you want to get the look in the image you posted.
To clarify, the screenshot posted appears to be using a combination of GlowIcons, which you will already be using if you have OS3.5/3.9 and MagicWB, which consists of the grey drawer icons and the volume icons you can see peeking out out from the left hand side of the "Sound" drawer. I believe the background used in the drawers is also from MagicWB.
The clock I think is the one provided with OS3.9.
With your spec you will be able to run OS3.9 with the above enhancements at a very reasonable speed.
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moto
