I wouldn't go that far. The MB is important, just not for performance.
The friends have a point about the components though.
The i5-2500 is a high-end gaming CPU
The Radeon HD 6870 is a mid-range GPU
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In total, they'll leave you with good, but not high-end performance, as the I5 is slightly bottle-necked by the GPU. Not by much and you probably won't notice it in WoW, but it's true.
Generally we say that an i5-2500 needs at least a Geforce GTX 560 Ti or a Radeon HD 6950 to feel comfortable.
Building a balanced gaming PC is not unlike the bank vault heist scene from Fast and Furious 5. If you have two differently powered cars, one will always have to slow down to accommodate the other, meaning you won;t get the most bang for your bucks. Think about it, you're combining a 200 USD CPU with a 150 dollar GPU.