I use a firepod with a computer with an athlon chip. No problems at all.
I've looked into the firebox for a friend, and it seems to be almost the same thing just on a smaller scale. The only thing that the box lacks fromt he pod is the "mix" knob. This allows you to mix your input signal with your playback signal (input would be what you are recording, playback is what is already recorded - that you're recording with). This is very handy because, say you are doing vocals and you've got everything else recorded. If you get your input set on your vocal channel where you want it, and you want to hear more vocals in the mix, just turn the mix knob towards inputs. If you want to hear more of the instruments you've already recorded, turn it towards "playback". Hopefully that made sense.
Anyway, kind of went on a tangent there - you could do essentially the same thing buy turning the channel volume up in your recording software, but its nice to have the feature on the front of the firepod.
I think the FireBox would be a great tool - especially if you don't need all 8 mic pre's and 10 inputs of the firepod. I think eventually if you did need them, you could daisy-chain them together anyway, so you'd be set.
My friend did get one on ebay the other day, and he found an ebay dealer who was giving away a free gift with it for $300 with no shipping. The free gift was your choice of some great gear. I think he got an MXL V67G for free with his. All the gifts were worth about $100. I think the dealer was Hollywood Audio" or something like that.
I'll see if I can't find it really quick.