Be careful with a NAS solution. The most important issues with a NAS system is speed of file transfers (as this will be your main operation that is performed time and time again with a NAS).
If you’re only serving windows machines, then go for the HP Windows Home Server. This is very fast on transferring files back and forth from other windows machines.
However if you need to transfer files between non-windows machines then you have a problem if you want to map drives and use the native file system of your NAS (which will be the fastest method).
You see Windows uses NTFS which MS have not released the source code or API's for (so, it’s secret). Thus Linux or Mac systems cannot map this file system properly (yes Linux does have a NTFS driver, but its beta/unfinished).
So, how do current NAS systems work? They use FAT32 which is a way older slower file system. And it will feel slow when transferring large files. But this way they can allow all machines to map to them.
This is also why HP's system always beats the compo as it's only benched marked at xfering windows files back and forth using NTFS. Whereas the others have full o/s compatibility but WAY slower xfer rates using FAT32.
So, what other solution can you have if you have multiple o/s?
You could build your own NAS using a standard Linux file system format that Mac's can see and Windows can with an open source driver (Samba). This way your not limited to FAT32.
You will find ALL NAS solutions on the market that can use Linux and Mac, as well as windows use the very slow FAT32. So, the solution is either:
HP Windows Home Server.
or
Build you own Linux NAS.
That's if you want to use your NAS for file transfers :P