If you're building a budget gaming system you can't just go by %. Say your budget allows for $600, and you can hypothetically make the lowest end system possible that will still play "game X"
Drop to $500 and all of a sudden you keep the same cost ratios, but the system is a total POS, unable to be upgraded down the line, and unable to run "Game X"
Instead of adhereing to a %, you need to consider them as components. You NEED certain components, regardless of how much % they take out of your budget. If your Socket775 mobo with PCIe all of a sudden becomes a Socket478 with AGP, because you were trying to stay on budget, you've just shot yourself in the foot.
With a shotgun.
Rather than think about "how much of the total" it is, you should think rather "how much is the total going to be now?"
Let me clarify. I think you're putting the cart before the horse. It's all well and good to hope for a certain budget, but if it doesn't pan out it's folly to scale back the machine to fit %'s. Either you can afford the video card, or you can't. Either you can afford the CPU or you can't. IF you can't afford one, you get a placeholder at a fraction of the cost, and replace it later when more funds come in.
Example: I bought my motherboard and vid card based on performance. The mobo supports 1033MHz FSB Conroes. Only, I couldn't afford the CPU. So I got one that *also* took my existing 800MHz FSB 2.6GHz P4, and upgraded that 6 months later. I didn't scale back the motherboard, or the vid card. If I had, the "upgrade" would have been more of a "sideways-grade."