Upgrading a 4 Year Old Build - Where to Start?

so here is your revised system listing ( I set all parts to 0.00 since you own them all

PCPartPicker part list / Price breakdown by merchant
CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor (Purchased For $0.00)
CPU Cooler: Cooler Master Hyper 612 PWM 82.9 CFM Sleeve Bearing CPU Cooler (Purchased For $0.00)
Motherboard: Asus P8Z77-V LK ATX LGA1155 Motherboard (Purchased For $0.00)
Memory: Mushkin Blackline 16GB (2 x 8GB) DDR3-1600 Memory (Purchased For $0.00)
Storage: Western Digital Caviar Black 1TB 3.5" 7200RPM Internal Hard Drive (Purchased For $0.00)
Video Card: Asus GeForce GTX 560 1GB Video Card (Purchased For $0.00)
Case: Zalman Z9 Plus ATX Mid Tower Case (Purchased For $0.00)
Power Supply: OCZ ZT 550W 80+ Bronze Certified Fully-Modular ATX Power Supply (Purchased For $0.00)
Total: $0.00

if it was my system these are the basic upgrades I would do, all depends on your budget of course, and if you play any games

PCPartPicker part list / Price breakdown by merchant
CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor (Purchased For $0.00)
CPU Cooler: Cooler Master Hyper 612 PWM 82.9 CFM Sleeve Bearing CPU Cooler (Purchased For $0.00)
Motherboard: Asus P8Z77-V LK ATX LGA1155 Motherboard (Purchased For $0.00)
Memory: Mushkin Blackline 16GB (2 x 8GB) DDR3-1600 Memory (Purchased For $0.00)
Storage: Samsung 850 EVO-Series 500GB 2.5" Solid State Drive ($219.99 @ Memory Express)
Storage: Western Digital Caviar Black 1TB 3.5" 7200RPM Internal Hard Drive (Purchased For $0.00)
Video Card: EVGA GeForce GTX 1050 Ti 4GB ACX 2.0 Video Card ($184.98 @ DirectCanada)
Case: Zalman Z9 Plus ATX Mid Tower Case (Purchased For $0.00)
Power Supply: OCZ ZT 550W 80+ Bronze Certified Fully-Modular ATX Power Supply (Purchased For $0.00)
Total: $404.97