I was gonna say, once Apple releases G5 xServes, you could just put 4 of those together, and have twice the ram, and a lot more storage, and a faster computer...
You can cluster 4 dual G5 PowerMacs to do the same thing now. They should take up about the same amount of space. They would also be more powerful than an Athlon MP based system if configured correctly. May not be able to compete on price, however.