For gaming there are two options.
Option #1: Build the rig yourself. Itís not as challenging as people think to do this, but itís a bit like riding a bike; or in other words, you have to have done it once or twice to get the hang of it. And if youíve never done it before, you would have to find someone to teach you how, probably by going through the process from back to front and literally looking over someoneís shoulder as they build one; or as a better way, get all the parts together and have someone direct you how to (carefully) put them together. If youíre going to build one yourself I have some advice that could be useful.
I highly recommend using MSI gaming G series motherboards, or ASUS gaming series (I believe theyíre specifically called ďRepublic of GamersĒ) motherboards. For the amount of memory for a gaming rig, it should generally be about twice as much (or more) than a standard computer (such as an office workstation) that you would find for sale at the exact same time youíre planning to build the rig would have. It should also by definition have a processor thatís a decent amount more powerful than say a standard office workstation would, and beyond that, the key to the rig is often the graphics card. The importance of this last component should not be underestimated and great care should go into selecting the GPU best fit to what you want. When it comes to gaming -- graphics cards can be everything.
Option #2: Buy a system from a vendor; and if youíre going to go this route it might be best to contact a provider like Alienware (which is owned by Dell). This is a more expensive option to be sure, and the reality is it leaves you less control over what parts go into your new system, but if you donít know how to build one and canít find someone to show you, then this is certainly the best choice.
For example of a hypothetical problem with option number 2, right now (at the time this post was written), Alienware doesnít have any AMD processor based systems available for sale. But thereís no real technical reason you canít have a gaming system with a high end AMD processor at all, and if youíre just dying to have AMD for some reason you may not like this. So you can see how buying one leaves the user a little less control over whatís in their new rig -- but still, in the end thereís nothing wrong with it.