You should build it yourself or find a place where they can build it with your specifications. No doubt about it.
Many big stores that sell branded computers will try to convince you to buy these computers by embellishing what they say, telling some tiny lies. Big enough to make you mad.
I once went to one of these stores, Future Shop (in Canada), to browse the alleys. I just decided to ask some questions about some computers already knowing the answers. It is then that I first realised how incompetent/unreliable some employees can be in these stores.
The computer I asked questions about was a HP Pavillon 1GHz I think. I asked a bunch of stupid questions like: Is this a good computer to play games? The guy said yes so I asked: So there is a good video card in it? He said yes and that I could play most games on it. Another question I asked was: So if I would want to upgrade it later it would it be possible? He said yes again.
All along I knew what he told me was BS. The computer had no video card whatsoever. It had 4mb of onboard video, and no AGP slot! The only slots left were 2 PCI and one RAM slot. The tower was so small, I dont think someone could add much in it. WOW thats one awesome gaming system!!!
I just had to tell this cuz it was funny...

Maybe I should have embarrassed him before departing.