It all depends of your money man

i got an old pc (athlon 650, win 98se , 450 Mo ram, DD 20 Go, Terratec DMX 6 Fire sound card). Now everything is ok but i've spent nights and nights trying to get the good settings, looking in forums for help by experimented users.
When buying the computer, check the motherboard, avoid VIA chipset as it's not really good for audio, you may get irq problems with the sound card. For the soundcard, maybe take one with digital i/o to record from mpc (with digital i/o) to pc, it improves sound quality. Check the asio drivers of the card (asio 2.0 at least) You'll may have to format the hard drive to install the strict minimum, as you'll have to save the most ram and drive place for music ! Get xp pro and cubase sx only if you have 512 mo ram, a fast processor (p4 1.5) and a big hard drive.
Generally, you won't need the latest and fastest computer on the market, but the newest it is, the easiest the installation of everything will be. As i told you, it's all about the money !
Little add : well i've got some noise with the soundcard, it's because of via chipsets, not good for audio argggg Avoid this kind of motherboard if you buy a pc !!!