Regardless of such requirements, the best desktop is one you build yourself because realistically, you're gonna spend a certain amount of money anyways so you might as well put 100% of that money towards parts rather than split it between parts and labour/warranty; that way you'll get more for your money.
If you're into "regular" OEMs, Dell has really gotten their act together over the last 6 or 7 years and are a nice choice. If you're into the "fancy" OEMs but have a lower budget, Alienware boxes can be had for quite cheap. If you're interested in "ultra high end fancy" OEMs, look to Voodoo