I can't see any reason why the motherboard would support the Q8400 and not the Q9450. It has the Q35 chipset which supports both processors. Both processors have the same TDP limits (95 watts). The only difference I can find is the amount of L2 Cache (4MB vs 12MB).
Personally, this system is so old that I would not bother spending money on an upgrade. If you can get either processor for free or next to no money, then I'd go ahead with the planned upgrade. Otherwise, I would not bother.