Always ask yourself this question when making a server, well at least I do.
 
1. purpose  ( File server, application, web, test box, folding)
2. cost limit or how much you want to drop.
3. length of life span / can I upgrade it later
4.  Home brew or a pre built server ( HP, IBM, Dell just to name 3) there are uses for each. really boils down to personal preference.
5. time willing to troubleshoot with the unit.*
 
*now 5 may not seem too important but honestly, I have found it much easier to get a used server, and just upgrade the parts to what I need. keep in mind taking almost any (dell, HP or IBm) server that is relitavly recent you can get the ram , and hard drives mostly cheap and keep it running for a lot less than a new computer. now that being said there is a steep learning curve if you never played with raid configs, and learning the finer points of hardware replacement and firmware updates on server grade hardware. Most of these time, they are redundant and hot swappable out of the box, so multiple components can fail and the server can keep chugging along ( Although this is not a desirable thing have happen it does save you most of the time for 24x7 up time)
 
Its just my opinion but I love rack mount PC's It is a space saver and a heck of a lot easier to work on quickly. I recommend some HP rack mount servers, depending on what you are using it for you can pick up some fairly cheap ones and build it to your spec. such as DL 320 G4's ( http://h18000.www1.hp.com...12365_na/12365_na.html )  they take Sata/SAS drives, and currently I have 3 running with 2 TB drives in each so 4 TB in a 1U. of course it is not officially supported by HP because your not using official hp branded drives, but they do work :-) just don't tell anyone I told you hahaha. you can even get SSD's to work in them. Ebay and Craig list are good places to get them as well as local auctions, most businesses are dumping anything pre Gen 5's and occasionally you can get your hands on g5's either DL 380's, or DL 360's, I prefer the DL 360's because they are 1U. Both support VT-X if your into virtualization like I am ;-).
 
Although It does depend on what  the server is preforming. Maybe you would be better just making a good computer based on a strong motherboard with tuns of Sata connections or a good raid card. we really need to prob get a better idea of the goal to guide you in the best direction. but if these  servers will work for you, just ask me any questions you have. I love them and its what i do.
 
Jacob