The size of the monitor doesn`t matter at all, its all in the resolution and im guessing its 1080p?
Anyway since your replacing the whole parts why not replace the case aswell
PCPartPicker part list:
http://pcpartpicker.com/p/3cTW7P
Price breakdown by merchant: http://pcpartpicker.com/p/3cTW7P/by_merchant/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($94.99 @ SuperBiiz)
Motherboard: MSI 970 GAMING ATX AM3+ Motherboard ($99.99 @ Amazon)
Memory: G.Skill Ripjaws Series 8GB (2 x 4GB) DDR3-1600 Memory ($59.99 @ Newegg)
Video Card: XFX Radeon R9 290 4GB Double Dissipation Video Card ($289.99 @ Amazon)
Case: Fractal Design Define R4 w/Window (Black Pearl) ATX Mid Tower Case ($89.99 @ Newegg)
Power Supply: SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply ($79.99 @ SuperBiiz)
Total: $714.94
If you wanted it to be a bit cheaper you could replace the card with a r9 280