I think Homeplugs are up to 200Mbit/sec so 200/8 = 25MB/sec, so even USB2 would be a bit faster - and I doubt homeplugs get that full speed in reality.
If you were to get a NAS box
http://www.overclockers.co.uk/productlist.php?groupid=46&catid=2125 on a gigabit network you could theoretically get network transfers of 1000mbit /8 = 125 MB/sec.
Of course you'd need a £400 NAS box PLUS hardisks, plus a Gigabit switch, PLUS cabling - that's why I went for a £30 external USB HD instead which I plug into my TV and lets me watch all my films that way!
