When I started to look what I needed from a computer to run a front-end and a couple of emulator, I found that I had all the required parts lying around just waiting to be assembled.
For the chassis, I chose to use an old Fujitsu-Siemens case with the front panel removed. I decided to keep the case and not just fasten the loose parts to the cabinet so that the parts would get some extra protection from outside damage.
The main board is an old Asus board with an integrated video card with 128MB of RAM. That was enough for any game I intend to run on this cabinet. The processor is a single core 2,2GHz Intel processor with enough juice to run anything. I put in three 512MB DDR1 DIMMs of Kingston 400MHz memory to get it running fast. I could have gone with less, but it never hurts to play it on the safe side. For the hard drive I used an old Seagate 80GB drive. It was way too big for the games, but it gives me an option to upgrade my system. Sound card is a Creative Sound Blaster 5.1 Live. Overkill, I know, but had one extra lying around.
The computer is also in charge of the cooling of the cabinet. I soldered an RJ45 network cable to run two 92mm fans to the top of the cabinet. I used one molex-to-fans adapter with two 5V and two 12V to give five volts to the fans. The soldered RJ45 has two three-pin male connectors in the computer's end and two molex connectors in the fan's end.
The power button is also taken a bit far from the computer. I soldered one RJ11 cable to take the two-pin motherboard connector to a microswitch in the back of the machine with the main power switch. This is so that I have all the power controls in one place.
For the front-end I chose Maximus Arcade. It seemed the most suitable for my needs. It also has good customizing options and skin editor. I tried a couple of the free ones, but found that many of them lacked some parts, or just weren't very pleasing to the eye.
Also for those who say that why pay when you can get one for free (for example MaLa Frontend), if you are paying possibly over a thousand euros for your cabinet, what is a mere 20 or 30 euros for a well working front end that does what you want it to.