First off, if you are going to go into the Game Server business, you should hold off and research a lot more, as to avoid going under early. As to the link you provided, that's overkill for game servers. There are some unnecessary 'extras' on that server, shooting up the price, that you do not need for running game servers. If you really had that much to spend/invest in your game server company, then I say great --at least you won't be the many gamehosts that only have enough funds to cover the month-to-month dedi charges, and go down when people run out on their bills.
As to colocation, colocation is 'locating' your server in a datacenter --something else you might want to read up on if you're not yet familiar. You'll also want to get familiar with providers and common terms used, since latency plays a pretty big role in game servers. Colocation is different from Dedicated Servers because in colocation, you actually own the hardware, but just sending it off to be housed, as opposed to Dedicated Servers where you just rent the server (already connected to a network) off of a host.
all you really need for a game server is a dualie setup, SATA or SCSI hard drives, and 1-2GB of ram. You don't need a FireGL ... infact you dont need a video card at all. Take the others' advice and do some more research.
You can put the same amount of slots on a $1500 server as your could that $6000+ one.
Too much ram, too large of hard drives, too many hard drives, too much power usage, and it's a tower and not a rackmount.
Keep researching, look for a Dual Xeon or Opteron with 1-2GB, 80GB HD or 36/73GB SCSI in a 1U or 2U rackmount case.
As the GSP hosts will tell you, the slots completely depend on the actual games you allow, the actual usage, and how many customers you put on that box. It just varies, some use more of one resource like CPU, others use more of a different resource like Memory or Bandwidth.
I would say your main focus would be on cpu and RAM.
Going with dual 3.0s or 3.2s would save you several hundred dollars over the 3.4s. For the ram I would recommend 2GB of DDRII 400 or whatever your board supports, at least faster than the ancient DDR 266. You only really need 1 harddrive, an 120GB SATA or 73GB 10k scsi (if the motherboard has onboard scsi) would be sufficient. Try to stay with a 1u or 2u rackmount as well, tower servers take up too much room and can be costly in the long run.
Buying the best hardware won't always give you the best game servers. Like others said, research more. Figure out what operating system you would like to run, how you are going to optimize the game server, what features you are going to provide (control panel, etc), and also very important is the datacenter you will be sending your server. Go with a quality bandwidth provider, cheap bandwidth will give you hundreds of compaints and customers that never stay more than a month.