This really depends on several factors. First and foremost is the length of time you plan on being in that facility. Obviously, short term data centers may be different than longer term facilities.
Broadly speaking, your options are single- or multimode fiber only (which is quite rare), a combination of fiber and copper (the most common) or copper only. Fiber will likely remain the medium of choice for backbone applications. Your choice in fiber is either 62.5 or 50 micron multimode in various grades (internationally also OM3) with the most popular being 50 micron laser optimized or you may also hear the term zero water peak. Some facilities build in an amount of singlemode for future proofing, but it will likely remain dark for some time.
On the copper side, your choices are category 6, 6A (shielded or unshielded) and category 7/class F (fully shielded S/FTP). As of today's date, category 7/class F is the only published standard that is called out in the IEEE 802.3an standard for 10GBASE-T. The category 6A (augmented category 6) standards should publish in 2007. Category 7A standards are also in progress although products for that standard are already available. Basically a 7A system is a 7 system with 1000MHz cable.
With copper, the longer the data center will be in place the higher the category of cable you will want to install. 10GBASE-T NICs have already been introduced with more on the way. So the likelihood of adding 10G copper components over structured systems is right around the corner. As we continue to condense servers with virtualization while gathering more and more information at the same time, our network needs will continue to grow. So if you are going to be around a while – plan big. Ripping out and replacement is expensive.
ABOUT THE AUTHOR: As the Global Network Applications Market at The Siemon Company, Carrie Higbie supports the end-user and electronics communities. She has won the "Communication News" Editor's Choice Award for the last two years.