Boy, that is quite the question!
Performance vs Cost? How important is graphics? Do you need/want a 'maintenance package?' Do you want to be able to work on it yourself if something goes wrong? For gaming, do want a gaming mouse/keyboard or is the one that 'comes' with the package ok? Do you want >5.1 dolby w/ surround sound, or are two low budget tin-sounding speakers w/ on-borad sound ok? Flat Screen, LCD Screen, or Boat Anchor monitor? I prefer AMD over INTEL, but honestly I its mostly preference... You'll want fast refresh on the monitor...etc.
I know that you said you don't want to, or your situation requires that you can't build it yourself... But you should look into it - because its actually not that difficult, and you could save yourself some money. If you get well supported equipment/components, and on the equip list for XP, it is as straigtforward as hooking together some components... imho
But I do understand, and know that there are reasons for not wanting to do build it yourself. There are a few threads going currently that discuss the pros and cons of some namebrand machines...check 'em out. But honestly, I wouldn't think a namebrand machine would be the "Best Gaming Computer." Unless you are rich and can throw money at alienware type equipment.
hmm...I apologize for not really answering your question..heh.