In terms of what computer specs you need, it's not too tough to build a rig for it. I use an i7 2600k, and I think Jaxel uses the same. However we both built our computers before the 3rd Gen Core processors were released, so now those are the standard. Basically just make sure you get something quad core from Intel, and make sure it still ranks good on sites like cpubenchmark.net. Honestly just about any normal current i7 will do you fine.
In terms of capture cards, I use a Blackmagic Intensity Shuttle, which is by far the best external capture card. Only problem is that it uses USB 3.0, and not just any 3.0, but only certain types (which isn't exactly advertised) as it's not compatible with all. It's tough to get your computer to work with it perfectly, but when you do it's awesome. To give you an idea about this, when I bought my new laptop I actually returned 2 different laptops because it ended up that they didn't work with it. They're going to be adding more compatibility soon, but it's not there yet. Jaxel used to use the Blackmagic Intensity Pro, which is essentially the same as I have but internal instead of external. And because of how it connects internally, the compatibility is MUCH better and easier to work with. I only went with the Shuttle because I needed it for my desktop and laptop. If you only want to work on a desktop, go for the internal. Jaxel's card recently broke though, and afaik he's trying to switch to a card by Avermedia, but I forget which one.