Gaming desks are top-notch systems that can run pretty much any application you throw at it. Whether you use it strictly for gaming, or actually do work in high-end application, one basic rule applies to what such a desktop should contain in terms of hardware.
The first thing you need to do if you're trying to figure out what to upgrade in your computer is to make a list of your current components along with the slots that fit in. If you note your CPU, it stays snugly into a socket. You'll want to jot down that socket model if you're trying to find a better CPU to go with your machine. Modern games can be pretty demanding, so it's wise that you should upgrade to a quad-core if you do not already have one. Try to find the perfect balance between price and power when looking for your processor. It should not be the most expensive one you can find, but one that can suit your everyday gaming needs.
The next item on the list is your system memory. Do you have enough memory to run your games perfectly? One thing is for sure when it comes to RAM, and that is that you can never have enough of it. Try to aim for at least a 4GB configuration. Take note on the occupied slots on your motherboard, and see how many are free to be filled. If, for instance, you have a slot with a 1GB RAM module, and three free slots then those can be filled with 3 additional 1GB modules to increase overall system performance. You should also check your motherboards manual to see the maximum supported system memory. That way you can get the general idea of what heights your system can be taken to.
What hard disk type do you have installed on your system? Is it a traditional 5400rpm model? If that's the case, then you should seriously think about getting either a 7200rpm HDD or the more advanced SSD drive. Solid State Drives are currently the best solution in data storage when it comes to speed, reliability and life-span. These drives are up to 2-4 times faster than regular HDDs, with faster data read and write times. The only downside of these drives is their price. Even if they do come at an increased price, you should take it as an investment in your gaming future. Once you get such a drive, chances are you will not need to replace it ever again.
Last on the list is your video card. A graphics card upgrade can get pretty expensive but it's well worth the purchase if you want to experience true gaming bliss. Jot down what you currently have, see if there's room for improvement and get to searching for a better model. One idea would be to opt for a system which uses multiple GPUs for extreme performance. These do not come cheap, since it requires you to buy at least 2 models of the same graphics card and a motherboard which supports such a setup. If you do manage to get this done, then you will not have to upgrade your system for years to come, since these configurations can run even the most demanding games on ultra-high settings.