I do have a problem about which OS would suit best on my NAS for quite a while. I had Lacie Ethernet mini, NAS device which is Linux Embedded, a while ago and that served me well. In addition, I need an extra one since that NAS device had not enough storage for me anymore. So, I have microATX box which installed Ubuntu. Yes, the performance gained significantly from ARM-based system to x86 system. My electrical bill, however, went up as you could expect since that was <20W on NAS to 45-50W on full PC. Then I came up with an idea of merge DVR with NAS together to save the bill. My only answer would be MS Windows again. It doesn't matter which version I choose; they are basically the same. If you ever tried Windows Media Center, you will know that you will be missing something without it. I think it might apply to whom you are using TiVo or similar stuffs as well.

The reason why I wrote this, nevertheless, is not urging everyone to use like I do. I just want point out pros & cons of each system as a NAS.

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  Lacie Ethernet mini Ubuntu Feisty Fawn Windows Vista Home Premium
Power Consumption lowest medium low-mediumw1
Throughput slow fast fastest
Protocol FTP, HTTP, SMB FTP, HTTP, SMB, etc FTP, HTTP, SMB, etc.
Indexing ability N/A local onlyu2 Yes, Windows Desktop Searchw2
Ease of use Very easy (on/off) Normal Easy
Cost Free Free $$w3

u2 - yes it does have Beagle, but any remote machine couldn't use its index at all. If you guys have a suggestion, I would love to hear.
w1 - although I use the same system as Ubuntu, Windows does have 3rd party like RMClock to undervoltage which is really help. I could drop about 10W or so without sacrifice the performance and stability.
w2 - For Vista client, you could just exploring to that NAS and find them. It'll use index on NAS automatically which is so fast. But I couldn't find client with XP+Windows Desktop Search work the same way.
w3 - It's not free, Vista Home Premium OEM is $99; depending on what you think if it's worth or not.

All in all, choosing OS is ultimately up to you need. If you need a DVR system like me, Vista would be very good choice plus you will get the best performance from you NAS and search server capability also. You, however, would find an only NAS device is a best solution for you if you just want to add extra storage to your home with easy accessibility. If you have any other thought, please don't hesitate to share.

Note: By the way, there is an alternative OS which is truly for NAS like Windows Home Server ($159.) I had experience when it's BETA. IMO, it's great really but the point is its cost is just too steep while Vista Home premium (OEM $99) could do almost anything I need besides automatically backup the clients. It seems to be a worth trade-off to me. For XP, I think it would perform great and, as I know, its index from Windows Desktop Search would be available to remote machine like Vista too.