Ironically, you are one of the few people who probably should be buying a pre-built gaming system.
So-called gaming systems almost always (not always, but ALMOST) have a very weak graphics card, as far as gaming use goes. If you want a decent GAMING system, you pretty much have to build one yourself. But as far as photo editing, serious writing, multi-tasking goes...most systems built for "gaming" would work PERFECT for that.
I usually recommend a multi-core AMD system, but your specific usage might favor an intel processor. The only thing an AMD system would work better at is photo editing, but an intel processor would still work fine for that, and an intel processor would be better for everything else you seem to want to do.
This is an interesting question, actually. I'll see if I can find a pre-built system and maybe suggest some parts to build one yourself...
Prebuilt: On Best Buy, for about $1150 (then add a monitor):
Dell - Studio XPS Desktop with Intel® Core™ i7 Processor
Model: SX8100-1986NBC | SKU: 9697488
That's one of the few gaming systems I've seen with a decent video card. It would make the perfect high-end workstation, which is what you really need. The video card is actually overkill for what you would be doing, but it will work great! :)
Build your own:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005
Corsair 650TX power supply
http://www.newegg.com/Product/Product.aspx?Item=N82E16811146061
NZXT Gamma case (any decent mid-tower case would do)
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115215
Core i5 quad core processor
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128425
Gigabyte P55 1156 USB3 motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231311
8GB of DDR3 1333 RAM
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148433
Seagate 1TB 32MB cache hard drive
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125310
Gigabyte GTS 250 Video Card
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754
Windows 7 Home premium 64-bit (you need 64-bit)
Logitech wireless laser mouse
Logitech USB keyboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16824236052
Asus HDMI Full HD 23" monitor
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106335
Retail Kit LiteOn Lightscribe DVD burner
There ya go. That was fun. This is a monster system, spec'd by me. All top-notch name-brand hardware. NO weaknesses. Video card isn't the greatest for gaming, but it's a heckuva bargain for a high-end workstation!!!!