400 frontside

yes

The GX260 uses a socket 478 Pentium. The FSB can be either 400 or 533. The fastest CPU is a 3.06G FSB 533MHz (Northwood) Pentium 4. This is the only Pentium 4 with hyperthreading that works in a GX260, provided the motherboard is a late version. The early versions of the GX260 motherboards do not support hyperthreading.

Doesn't the GX260 mobo die if you try to update BIOS with a HT processor?

Mine came with a 3.06G and BIOS already installed for HT.