The Paladin :
no you can sell and build a new one... but I don't know ho much you may get for your old pc , maybe 200-300$ at best
so I don't know what your new budget would be for building a new computer..
i have a $400-ish budget.
would the parts you mention earlier(below) run games at 1080p 120fps?
AMD A4-3420 2.8GHz Dual-Core OEM/Tray Processor
ECS A75F-M2 Micro ATX FM1 Motherboard
Kingston HyperX Fury Black 4GB (1 x 4GB) DDR3-1866 Memory
Seagate Momentus Thin 500GB 2.5" 5400RPM Internal Hard Drive
EVGA GeForce GTX 750 Ti 2GB Superclocked Video Card