mac.
i don't know any professional photographer who uses a P.C. yes there are some out there i just don't know any.
when you start using different bits of specialist software the mac stuff always comes out first or P.C's aren't supported at all. the mac/design /photography thing started years ago because of the way you could allocate more ram to PS with a mac and the different way macs worked with colour profiles and the monitors look up table enabling the computer to tweak the RGB channels in a crt monitor in real time so you could see colour changes instantly.
things have moved on since then but you still can't run 2 monitors with 2 individual calibrated profiles with a PC without a resulting to a fancy graphics card and geek level software tinkering.
even my 'consumer' macbook runs an extra screen with it's correct profile and it only has inbuilt graphics no extra card.
P.C's often only have 4pin firewire which is useless for shooting tethered with digital backs that use 6pin powered firewire.
you don't actually need a fancy graphics card for photography, fast frame rates are for video/gaming.
some of these things may not be an issue now but they will be further down the line with your work.
mac.
i don't know any professional photographer who uses a P.C. yes there are some out there i just don't know any.
when you start using different bits of specialist software the mac stuff always comes out first or P.C's aren't supported at all. the mac/design /photography thing started years ago because of the way you could allocate more ram to PS with a mac and the different way macs worked with colour profiles and the monitors look up table enabling the computer to tweak the RGB channels in a crt monitor in real time so you could see colour changes instantly.
things have moved on since then but you still can't run 2 monitors with 2 individual calibrated profiles with a PC without a resulting to a fancy graphics card and geek level software tinkering.
even my 'consumer' macbook runs an extra screen with it's correct profile and it only has inbuilt graphics no extra card.
P.C's often only have 4pin firewire which is useless for shooting tethered with digital backs that use 6pin powered firewire.
you don't actually need a fancy graphics card for photography, fast frame rates are for video/gaming.
some of these things may not be an issue now but they will be further down the line with your work.