Can I add a network hard drive to my Wi-Fi net?
There are two basic ways to add storage to your wireless network, but in either case, it's best to physically locate the drive(s) next to your router and connect them by wires rather than using a wireless adapter. Generally, you needn't put a network drive in a different room, and a wired connection is always faster and more reliable than wireless, especially if you have gigabit-ethernet equipment.
What you are really looking for is access to your network storage over your Wi-Fi net, which you can achieve by connecting any Network Attached Storage (NAS) device to one of your router's ethernet ports. Alternatively, you can buy a device like the Linksys Network Storage Link NSLU2, which connects two USB hard drives to any router via ethernet.
Can I use VoIP over Wi-Fi? What kind of quality will I get?
Voice over IP actually requires comparatively little bandwidth--under 100 kilobits per second per call--whereas network throughput is normally measured in megabytes per second. The problem with VoIP over Wi-Fi is more an issue of priorities: If someone else on the network is downloading large files from the Internet at the same time that you are making a call, choppiness and delays can occur.
Although the faster your router is, the fewer problems you should have using VoIP, most late-model wireless routers also incorporate a technology called 802.11e, or QoS (quality of service), that prioritizes streaming data ahead of regular data transfers. Be sure to get matching adapters that also support QoS, however.
How do I stream audio and video from one room to another via Wi-Fi?
Any audio or video that you can stream over a wired net, you can also stream via Wi-Fi. You just need to be sure that your Wi-Fi equipment's pipes are broad and fast enough to handle the data. For high-quality video, you'll probably need either 802.11e or a vendor's proprietary implementation of QoS enabled in both your router and adapters.
To stream your media, you'll also need some kind of streaming server, such as a Windows Media Center PC; an NAS drive with software like the open source SlimServer; or one of the many dedicated wireless streaming-media consoles, such as the D-Link MediaLounge Wireless HD Media Player or Roku SoundBridge M1001. See "Get More Out of Your Wireless Network" for more on wireless streaming.