I just finished my first build and now I am wanting to go with an htpc and I also have a small laptop. all I want to know is does the cpu on the server need to be equally as powerful as the cpu on my home build(I7 950) and the same with the memory(24 gigs). will a low power cpu and less memory on a server bottle neck my other computers. All I want is to store text and media files on the sever and have access to them from the other pc's, to free up space because I have four 64 gig SSD's in raid on my home build for OS and programs, and dont want to block airflow with bulky storage.
You should be fine with something ATOM based or even a low power AMD processor with a half decent motherboard. I'm planning mine now, But I know my friend has an Athlon II X4 605e with a MSI 760g motherboard. He says the processor is overkill but he does some Video encoding/transcoding on it w/o a graphics card so that comes in handy. If I were you, I'd pick up a low wattage processor (Dual core AMD or one of the low power Athlon II x3 or x4 models- They should have an "e" suffix and run on 45w.) Put a quiet heatsink and fans on it then you should be fine.
For a file server you can use a P4 with linux on it.
Not that you must go do it its just to give you a indication that you don't need a lot of cpu power power. What matters is your storage and the config. So going with a raid option will be a good choice. If you want to boost the disk performance get a Pci-e add in raid controller.
personally I would get a x4 955be. They're going for cheap and you'll have a allround system you can use for other tasks as well. If your looking for something cheap and anything that will do get a x2 for around 60usd.
4gb kits are going for cheap and the price difference between 2gb and 4gb ain't that big and it would be better to get a 4gb kit for the price they're going.
You can pick up 4gb gskill DDR3 ram for around 40usd
You CAN use a P4 but I wouldn't. Those things are power hogs. a Sempron 140/145 will do the job just as well.