Modular is nice to have but not necessary, It helps a lot with cable management as the cables you don't need don't get just hanging there, but not really much more.
As for your build, you're not getting an unlocked processor so you won't be able to overclock it should you want it so, either get a i5 4670k or then you can downgrade the motherboard.
Also, 12GB of RAM is a weird number to have, I suggest getting just 8GB (2x4GB). With the savings in the RAM you can get a better GPU, maybe a R9 270x or a GTX 760.
Finally, I think you're overpaying with that PSU, you can get a cheaper one:
So, here's your build with some modifications:
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor ($214.99 @ NCIX US)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.98 @ OutletPC)
Motherboard: Asus Z87-K ATX LGA1150 Motherboard ($112.99 @ NCIX US)
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1600 Memory ($64.99 @ Newegg)
Storage: Samsung 840 EVO 120GB 2.5" Solid State Disk ($78.99 @ Newegg)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($82.99 @ NCIX US)
Video Card: Asus GeForce GTX 760 2GB Video Card ($249.99 @ Micro Center)
Case: Corsair 300R ATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($49.99 @ NCIX US)
Optical Drive: Asus DRW-24B1ST/BLK/B/AS DVD/CD Writer ($19.98 @ OutletPC)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Monitor: Asus VE247H 23.6" Monitor ($139.99 @ Newegg)
Case Fan: Corsair CO-9050017-BLED 66.4 CFM 140mm Fan ($11.34 @ NCIX US)
Keyboard: Corsair Raptor K50 Wired Gaming Keyboard ($69.99 @ Newegg)
Total: $1256.18
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-04-18 22:36 EDT-0400)
Basically I downgraded the case a little and the RAM, and with that I got an unlocked CPU, an aftermarket cooler and a better GPU.
Also switched the PSU for an equally good one, although not modular.