you should build a computer. First buy a case with a 400w power supply and that has one or more fans, and also has 2.0 usb. Buy a good mobo(mother board) with 2 or more ram(random access memory, but mostly known as ram) slots, has a spot for pci express slots for a video cart, you need to get a possessor with the same socket type(with has to match), you need special type of glue for the possessor to be placed onto of it and under the fan,the glue i used was heat sink, go to a professional computer specialist to place the glue on RIGHT, buy a good video card that matches the mobo specifications(try to get a 6000gforce or more,get a sata hard drive that is 80gb or more(size mostly depends on what you putting on the hard drive), also get a dvd drive.. i would get every thing from newegg, a online store at newegg.com, if you don't want to have a chance with the part not working together then go to a computer store and ask someone for help.you would probably speeds about $500.or you could buy a premade computer from almost any electronics store, but just keep in mind on what i said.
Me crash169 could of missed stuff that you need to buy or imported detells on stuff to do and cannot be held responsible if the computer does not work.
It depends on where or what are you going to use it. Is it for office or business use or is it for your personal or family use? When you use it often and requires you to maintain a lot of file, then buy a high memory computer.