Well, if you just have the E6300, it depends on what kind of server you need. If it is just used for a few files and relatively light use, a standard desktop will work. If you intend to use the thing as a media server, store a lot of files (hundreds of GB) on it that you don't want to lose, or otherwise need a machine to always be there, then you want a *real* server. Here's what you'd need for a real server:
1. A suitable socket 775 motherboard. You can use a regular desktop motherboard, or for better reliability (but no overclocking which is completely inappropriate for a server) go with a socket 775 server board with a server chipser like the Intel 3000 or 3010. The Tyan S5197 looks like an excellent LGA775 server board but it'll cost ya.
2. Sufficient DDR2-533 RAM. If it is a game server, then you might need 2GB, otherwise 1GB is more than enough for a simple file server.
3. Enough HDDs. Be smart and plan to put them in RAID 5, so that means 3 or more HDDs. The Seagate NL or WD Caviar RE2 make good "bit bucket" HDDs. After you've bought as many HDDs as you want, buy one more to use as a (cold) spare so you don't have to run your array in painful degraded mode while your replacement HDD is shipping.
4. Two identical PCIe or PCI-X RAID cards, dependent on what your motherboard has for slots. Stay away from PCI cards as the only ones that won't bottleneck are 64-bit 66 MHz PCI cards and those slots don't exist on new boards- they've been replaced with PCI-X and PCIe. Get two cards because if you only get one and it does, you need to find an IDENTICAL card or else your data is toast.
5. A case with sufficient room for your hard drives. Look for one with good cooling as this machine will probably be on 24/7.
6. A good-quality PSU with enough SATA power connections. It doesn't need to have an extremely high watt rating as the CPU and motherboard don't draw all that much and there's only an IGP. (Don't put a GPU in a server.) Each HDD needs something like 12 watts, so figure accordingly. Quality is more important than capacity in this case.
EDIT: I see that you say you have the OS. Which OS is that by chance?
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