Don't worry. That's easy.
1. Download DarkCore. Click here. Thanks to DarkyRom Team especially Ficeto
2. Download ODIN. Click here
3. Run ODIN and flash DarkCore "DarkCore_4.0_JVS.tar"
>Make sure all KIES process terminated/killed. Check Task Manager
>Your phone must be on download mode when flash using ODIN (Power+Volume Down+Home button)
DONE. Your I9000XXJVS rooted!
Let's try a week of stock ROM 2.3.5 + DarkCore 4.0 :)