I'm thinking about getting a Xeon for a cheap build because I can't find any Core 2 Quads on Amazon for less than $10. Do I need a modded bios to support a Xeon with the 775 mod stickers?
Generally speaking, yes. You'd be better off checking this because modded CPU knowledge is seemingly rare here.
From my own previous investigations, you need to know whether your motherboard is capable of supporting a modded Xeon and whether the microcode is available for it. Thoroughly research the variables before going through.
Generally speaking, the microcode is needed because the modded Xeon requires it. It is also what you need to find out if such a thing is available for the motherboard you are using. The link I provided has far more information I'm able to provide and more comprehensive. If memory serves, they also list which motherboards are likely to work with which modded processors.