The link is from the i9000 and hope that someone made custom ROM for i5800 by using this.
Originally written by : Nokia7Vista
It can soft brick the phone.. Meaning the phone may not boot past the i5800(i) screen. If you see that screen, it means that you can't get past the Second Boot Loader (Sbl.bin). Reflashing with a good zImage should fix that problem.
That screen usually means there's an issue with the way the kernel was flashed, or the kernel/initramfs (both are contained in the zImage).