ARM has strange limits on the amount of memory. The Versatile architecture supports up to 256 MB max. The Versatile Express architecture has a 2GB limit (and no PCI!)
Then you have to remember that ARM is presently 32 bit, so you're going to have trouble going over 4 GB at all, which is currently a very small amount for a serious server. 64 bit ARM isn't going to be widely available for at least another 2 years, and at what cost we just don't know (but it's not likely to be £60/server).
The Cortex-A15 can address 1TB of ram, its still 32bit so there is the requisite per process limit. Last I heard Cortex-A15 SoC's will be available in volume this year.
Then you have to remember that ARM is presently 32 bit, so you're going to have trouble going over 4 GB at all, which is currently a very small amount for a serious server. 64 bit ARM isn't going to be widely available for at least another 2 years, and at what cost we just don't know (but it's not likely to be £60/server).