First things first, what made you decide on RAID3? It's not really heavily used. RAID5 is much more common and more or less does the same thing.
You don't actually need to purchase a Gbit router. You could simply purchase a Gbit switch instead, and then connect your devices to the switch. This would most likely be a less expensive solution, and would yield the same results.
Your server won't need to be overly powerful. If you plan on transcoding your media, then you'll need a bit more CPU processing power. Any dual core should work just fine. If you're worried about power consumption, the i3-2100T is quite powerful and has a TDP of only 35W. Samsung and Hitachi make excellent high-capacity drives as well. 4GB would be plenty, and for the price would be worth getting for a bit of headroom.
Cat6 will be plenty for streaming HD. You can actually stream HD content on a 100Mbit network as well if need be. Cat5e would work fine as well (still supports Gbit), but you may as well just go with Cat6 for a bit of future proofing.
You'll need to decide which operating system you plan on using before we can suggest any software as well. I would go Linux personally, but that's just what I prefer using. If you go with this method, you could use something like MediaTomb to stream content to your devices. There are alternatives of course as well. If you go with Windows on your server, I won't be of much help
Good luck, hope I answered some of your questions!
If i want wireless in my house too I need the router, for laptops and stuff. Also how do I even use the server.
Do I need a graphics card at all?
cat6 isn't that expensive so i'm cool with that.
also I'd like to run a linux server, I'm fairly familiar with terminal It's just I have no idea how to stream data from my server to my PC using mediatomb, and what about back ups. Ideally I want the server to create back ups of everything everyone does.
Also how do i control permissions? via the router or the server?
In terms of setting up raid 5, are there any raid cards you can suggest?
i'm looking at the Highpoint RocketRAID 620 SATA 6Gb/s Raid Controller Card - PCI-E.
here is my build I plan on thus far, any help would be great:
Intel Core i3-2100 3.10GHz (Sandybridge) Socket LGA1155 Processor - Retail
Hitachi Deskstar 7K2000 2TB SATA-II 32MB Cache 7200RPM - OEM (0F10311)
Gigabyte H67M-UD2H-B3 Intel H67 (Socket 1155) DDR3 Motherboard - (Sandybridge) ** B3 REVISION **
Highpoint RocketRAID 620 SATA 6Gb/s Raid Controller Card - PCI-E
Antec High Current Gamer 620W Power Supply
Sapphire ATI Radeon HD 6450 1024MB GDDR3 PCI-Express Graphics Card (11190-02-20G)
Cooler Master Elite 370 Case - Black
OCZ Gold 4GB (2x2GB) DDR3 PC3-10666C9 1333MHz Low-Voltage Dual Channel Kit (OCZ3G1333LV4GK)
Pioneer 24x Internal S19LBK DVD Rewriter including Labelflash SATA Retail
Sorry if I was unclear. You use your current wireless router (I was making the assumption that you already had one) in conjunction with your switch. Your configuration would look similar to the following:
ISP --> Modem --> Router --> Switch --> Devices
If you're using Linux, you can use the built-in software RAID, which doesn't require a RAID card. You could also use a RAID card for hardware RAID, although this solution tends to be more expensive.
Mediatomb has excellent documentation, and there is tons of community support for it. You basically install it and edit a single configuration file, which it outlines how to do on their website.
There are software utilities available for automatic backups, but I'm not too familiar with these. Hopefully somebody else can be a bit more helpful when it comes to this.
When you speak about permissions, it kind of depends what you're talking about. Generally this will be done server-side, but there is the chance you may need to do some router configuration as well. It just really depends on what you do.
You will not need a video card for a server either, as you generally have it running headless (without a monitor/keyboard/mouse) and access it through SSH/VNC/etc. from another computer. It just passes data over the network.