Actually I meant an unexpanded Amiga. But even if I take it literally, it's still true. I'd guess that 90% of your 880GB of data is not Amiga-only. Downloaded files, recorded videos, photos, mp3s, paintings, other creative data, all that usually needs to be accessed by several people on several computers. So it's a better idea to store those things on a network drive rather than locally on one computer. And using an Amiga as a network drive is only frustrating rather then useful IMHO because of its limited speed, network-wise, HDD-wise and CPU-wise.
I am using a PC with WinXP and 4 x 640GB configured as RAID 5 as a network drive. It's got gigabit ethernet. Accessing it from another PC is like accessing a local drive, no noticable speed difference. And accessing it from an Amiga is still fast enough to do software development with source and object files stored on the network.
WinXP is the right choice for a network server because it's the only OS which can be accessed by both, modern Vista-oid systems and ancient SMB implementations like used for AmigaOS.