Stephen you got yourself two very nice answers so far.
You know I am not a MacLover but do have several Mac at home just to be able to give my half a penny advise.
I just looked at my local shop and no surprise a full feledge mac 8core is just south of 5K US $ that I prefer to buy a second hand car at that price.
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Now remember that stepping into a new platform comes with its hassles of not knowing the very fundamental things that can be very frustrating.
Other things you want to consider is that a benchmark test is only good as a pop up ad on your pc. If you believe the ad, then believe the benchmark as these tests are so easy to be tweak to the direction of giving the favorable response by the pre sets of conditions set by the cleaver (Not so honest or politically correct, business minded) technicians. Depending on whom you are reading, you can get any kind of answer the designer want you to believe. So don't hope high when seeing an easy to understand, appearing end result and surely have a grain of salt to analyze who is saying what and how much you can believe.
Frankly if you need to know the bottom line, Photoshop runs faster on PC platform of equal horse power than Mac and video editing softwares (Especially the render part) runs faster on Mac than PC (Assuming you are using FinalCut on Mac as opposed to Adobe premiere pro, that is a different story and debate). I have done very successful video editing under both platform and to me the difference are slight (Same as photo editing process). If your primary intention is to use photo editing software, then you may actually do yourself a disfavor to switch to Mac. Now don't forget you may need to buy yourself a lot of new software, as your current pc softwares are not compatible, yes you can run them under windows emulation but they question is why.
If you just add ram to your new mac, Get ready, you loose your warranty on machine. Mac doesn't want you to be smart, buy your part cheap, upgrade or repair for very cheap as it is the sky that is the limit in pc world. You take it to Mac dealer for anything in the other hand and be generous to pay the bills. Absent minded costumers make a better candidate when thinking of the figures on bill. Every accessory in mac is 20-100 percent more expensive than equivalent in pc. The new power macs are actually very nice in terms of ease to open the case (Did I mentioned that they are really silent too in case you put them in the bedroom so you should not get a complain from spouse) but I am still to figure out how to open the case of my powerpc mac 6500 as there is no screw, no way to logically make you think you can open it (Just to change the DVD and ram), well with my angry wife on lack of space at home, I am determined to break it open just to see the R-rated version of mac before binning it.
Make sure to talk to some of your friends who own and run your intended mac and figure out if really it worths it.
I have put most of my pcs together myself and always the specification has best suited my needs at least tag price. Checking the Dell's outlet, the top of line XPS with water cooler sink for overclocking, Mega mega ram storage where much better priced when compare to mac but well then this is your money, spend it how it may please you best.