Buy a Mac.
I may not be particularly biased by the Canon vs. Nikon debate. But I am COMPLETELY biased in the Mac vs. PC debate.
I'd get a 27" iMac... get the higher end model (it'll have a better graphics card). Pick any amount of memory you want (memory is a snap to upgrade. My Mac has 16GB but you can get them up to 32GB these days.)
I DO HIGHLY recommend the "Fusion" drive. This is a special hard drive which is a hybrid SSD and traditional hard drive. The OS intelligently moves apps and files to the SSD if it notices you use them a lot. Stuff you hardly ever touch stays out on the traditional drive. You can get a 3TB fusion drive for $400 and in benchmarks it's extremely difficult to tell the difference between a full SSD vs. a Apple Fusion drive (except in a true SSD, you can't find one bigger than 1TB and they cost a ludicrous amount of money.)
If it were me, I'd go for the 3TB fusion drive. If you shoot a lot then you'll quickly fill a 1TB drive. You can always get external disks, but I'd probably go for the larger internal capacity before having to get external storage.
Unless you are already a Lightroom user, I'd _very_ seriously consider using Aperture instead. The two programs are roughly on par, but Aperture is significantly less expensive (Apple's software is always less expensive than it's competition.) Aperture is $79 and that license is valid for every Mac that _you_ legitimately own. Lightroom is $150 and that's a single use license only. I do use Photoshop, but it's rare. Almost everything I need to do can be done in Aperture (this is true of Lightroom as well... you'll find the vast majority of adjustments can be done in in LR and you hardly ever need to invoke Photoshop.)
I can go on and on about the reasons I will only ever recommend a Mac and (at least until something changes) wont use a PC. I do have and use a PC. I'm a power user for both platforms. But for my personal use and when it's my money, the Mac is so much better (the comparison is not close.)
If you are interested in specifics, I can have a practically unlimited number of examples.
I've helped a LOT of people with computer buying decisions. An extremely common thing I hear AFTER they've had the computer for a few weeks is "WOW! Why didn't you MAKE me do this YEARS ago!?" (I've heard that feedback at least a half dozen times.)