As I can see, Safari 4 beta is not working just with a very simple 5 year old PC built with an AMD Sempron 2400+ processor with SiS 650 video chip cast on an ECS 741GX-M main board. It is also not working with OS 10.5.7 beta. When I installed it on my computer, its look was closer to Google Chrome than Safari, and much less like the new Safari 4 (see picture below). Apple "cretinely" advises in a very hidden place with small letters that the new internet browser must be installed on computers with offboard video cards. However if one takes a look at the basic information found in bigger letters, any an older PC can run the such software. Mine has DirectX 9.0C installed, the share of memory was increased, and nothing helped. It seems that Safari 4 beta demands more than offboard video cards, it demands some fixes that shall permit it identify the presence of DirectX. Apple didn't mention if the software demands on instructions like SSE3 and SSE4. My processor just manages files that depend on MMX, 3DNow!, and SSE (1) instructions. So I had to reinstall the computer because after I uninstalled Safari 4 beta, it didn't permit me to install any other previous version and it ate some files from Windows that made it loose over 80% performance. "Thaaaaaaaaaaaankyyyyyyyyyyou Apple!"
Last but not least, the promised effects don't work on Safari 4 beta for Windows. So my hope to use the upcoming versions of Safari is in the migration to a Mac mini which the latest versions are phenomenal.