Well in this case I recommend you to buy a mobo (motherboard) that supports most recent memory technology such as DDR3 and has at least one PCI express slot, 2 dual channel enabled RAM slots (if you can find a one with 4 slots that'd be great), onboard HD sound, gigabit LAN, SATA III ports, USB 3 ports and specially look for at least 1 IDE port because if you ever have to connect old hard disk or a CD/DVD drive that would be a big problem. And keep in mind that many recent mobo manufacturers target their recent products only for recent and future products so you have to search more than one store if you want to have a good configuration like that (look for Biostar, Asus, Intel, Asrock, Gigabyte mobos but do not go for cheaper ones like Foxconn, Mercury).
Then look for a good graphic card because I think you already know that onboard VGA sucks. So take a look for a decent product built with an AMD Radeon HD series chip or Nvidia Gforce GTX series chip. But in this matter if you want more performance go for Nvidia solution though always mind that in my experience I knew that Radeon chips are better with quality than the performance (XFX, Gigabyte, Sapphire, Asus, Zotac those are good brands).
After that you have to buy dual channel enabled RAM bundle (at least 4GB bundle) because when it comes to gaming you can feel a great performance by installing same type dual channel memory into your PC. Kingston RAMs are good but if you have some more bucks in your hand look for Patriot, Corsair or Gskill memories because they are great for gaming.
Then go for a hard disk possibly a SATA II 500GB or more because storage is very essential nowadays.Though if you seek more performance for super speed gaming and graphic designing you can install a high speed SSD drive as the primary HDD so then you can install the operating system on it but they are so expensive (mostly a 64GB one cost around 150 dollars).
Now it is the time to look for a good casing so find a strong one made with aluminum and plastic because if you can find that kind of one it will last long for many years without corrosion. And you can find a good one with many facilities such as many USB ports, memory card readers, fan speed controllers and so on. But especially check it's ventilation system with good fans/cooling system because as it affect for temperature inside your PC it also guarantees a good lifetime too. Then the most important thing comes up which is you have have find out a good power supply unit that best fits to your PC so always go for something like Cooler Master, Antec, Corsair or any branded one which has 80+ certification on it because if you buy a cheap china made unbranded PSU it will not guarantee your PC for at least 6 months. With a good quality branded PSU you can ensure your newly built systems lifetime for at least 3 or 4 years without any physical damage to it's peripherals in it.
After all when you going to buy a monitor specially look for a modern one such as widescreen (16:9) HD LED with 18" or more inches because when you are going to watch a new movie definitly you can feel the difference between an old one with that.
Finally you can buy anything that suits you such as a good sound system (do not buy cheap desktop speakers so lookout for a subwoofer system) and comfortable keyboard and mouse (for the keyboard and mouse I recommend you to search for Logitech brand because no one believes that I am still using my old KB/mouse for 9 years already).
So that's etc but do not forget to use an UPS or a surge protected power protector/multi socket for your PC for the safety.
Good luck and thanks for reading.
(This article is kinda old because I wrote it a year ago but I hope still you can find it useful anyways)
