I can't recommend Windows Home Server highly enough.
You want backups? It will give you the ability to do a "bare metal restore" - none of this "Install Windows, then all the applications like Office, then restore my files". If your PC has a hard drive failure, once your machine is fixed, you put the recovery CD in the drive, start it up and after a couple of questions, go away for an hour or two and come back to a completely restored PC.
You want NAS? How about being able to combine all your disks into one large volume? No more wondering which disk you put a certain file on - especially handy when you're dealing with thousand of pictures or hundreds of video files. Looking for safety? Turn on File Duplication and it'll make sure that you have duplicate copies of every file so that a drive going bad in your server won't matter - there'll be another copy on another drive.
Virus/Malware? I guard my PCs for that - my WHS box has never been attacked.
Power efficiency? There are WHS boxes that run on Atom chips. WHS has very low requirements and using "Green drives" keeps your power consumption low.
Performance? Well, gigabit ethernet would help. I stream HD content all over my house from my WHS box and not every link (yet) is gigabit and I've had no problems.
You get remote access as well. Download your files, even log on to your desktops at home through your WHS box when you're on vacation if you like.
The one caveat is that "Vail", WHS V2, is coming out soon so you MIGHT want to wait for that. There isn't going to be an upgrade path from what I'm reading (WHS 1 is 32-bit, WHS 2 is 64-bit).
The best part is that you have no idea what you might want to do with a WHS box in the future - and it will let you grow. Buying a RAID array mean that's all you have. Buying a machine that has, at it's roots, Windows Server 2003 (V2 comes from Server 2008 R2) means you're not locked in. You can put more software on it. It already comes with a fully functional web server. There's a pretty healthy add-on market (lots of freebies too) for WHS.