it's very dependent on the chip, athlon 850 could be in several diff flavors, as Deliximus said, possibly an 850 socket a t-bird, or a slot-a t-bird, or even a slot-a classic. the classics will run ok with via or other chipsets, the slot-a t-bird requires amd's irongate chipset. let us know what type of chip it is, if it's a slot and you'll post the codes of the top edge of the casing i can tell you if it's a bird or a classic.
J.