1. Chipset is important if you're going to use that as the RAID controller. Most southbridge-based (i.e. motherboard-based) RAID is not very reliable or robust. I have seen reports that the NVidia motherboard RAID implementation does not handle volumes > 2TB. If you're going to use a 3rd-party RAID card, the most important point (obviously) is to make sure the motherboard has a compatible expansion slot. If your RAID card is a PCIe x4, for example, you need a free PCIe x4, PCIe x8, or PCIe x16 slot on the motherboard that is able to accept a RAID card. (Some PCIe x16 slots on some motherboards will work with a video card only, and won't recognize a RAID card).
Chipset can also be important when considering throughput. Desktop chipsets (945/965/P35/X38/etc.) are designed with the consumer in mind, not a server. A server chipset like the Intel 3000 is better for this task. However, I have run servers on a 945/965 before and they run OK.
2. For a file server, processor speed is not very important. You can easily get away with the lowest end Core2 Duo (E4400 I think). Memory is more important, especially if you're serving large files. You should plan on 1GB, maybe 2GB.
3. I'm using an Asus P4M2 for my home server motherboard. It uses an Intel 3000 chipset, takes a Core2 Duo (rather than most server motherboards which require a Xeon), has on-board ATI video (so no video card required), can take up to 4GB of RAM, and has a PCIe x16 slot and a PCIe x8 slot (both are x8 electrically). The plain P5M2 is hard to find now, you may have to get a P5M2-E ... check the specs carefully, some are different.
4. Ask 10 people about RAID card recommendations, you'll get 10 different (and valid) answers. I like enterprise-level RAID cards from Adaptec, 3Ware, and LSI. My particular favorites are the 3Ware 9650SE series. Plan which RAID card you buy based on the number of ports you think you're likely to need in the future. i.e. You have 11 bays in your case, don't buy a 4-port card.
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Get a 12-port (or a 16-port, if you later want to move it to a bigger case), if you can. Get the batter backup module for the RAID card as well.
5. Use all the drives in a single RAID 5. There is no reason to split your RAID into two different arrays. Plus, you lose a second drive's worth of space for parity information like that.
Use enterprise-series drives. This means Seagate Barracuda ES.2 (or older ES series). These drives have higher specs for reliability, and have server/RAID-tuned firmware. You may have problems if you attempt to use ordinary desktop drives in a RAID of this size (liek Seagate Barracuda 7200.11s).