The 2003 desktop components would probably be to outdated for a viable build, except for the case and fans possibly, and you can definitely use the hard drive if you wanted to. So we could use the hard drive to save the $50 right there a new hard drive would cost. The old hard drive won't be the best, but that is money saved for now. We can also save $15 if you can reuse the cd/dvd drive from the old computer also.
For $400, a dedicated graphics card would be to difficult to price in. I mean, one that would actually be worth adding in would be to expensive for that price. Is the Windows cd(s) you have for 32 bit or 64 bit versions?
The easiest way to say the difference between APU, CPU, and GPU is this:
CPU - Processor
GPU - Graphics card
APU - A processor that has a graphics processing built in. The graphics processing for an APU is not as good as a GPU, but can be low to low-mid graphics card power.
This is a $440ish so it is a bit over your budget, but does have a dedicated graphics card and a decent system overall. You would need to reuse the cd/dvd drive from the old system and the hard drive too to save money there. of course using the operating system disc for Windows 8 will help save money too.
http://pcpartpicker.com/p/9Lhvqs
PCPartPicker part list:
http://pcpartpicker.com/p/9Lhvqs
Price breakdown by merchant: http://pcpartpicker.com/p/9Lhvqs/by_merchant/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($109.99 @ Amazon)
Motherboard: ASRock 960GM/U3S3 FX Micro ATX AM3+ Motherboard ($54.99 @ Micro Center)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1866 Memory ($76.99 @ Newegg)
Video Card: XFX Radeon R7 260X 2GB Double Dissipation Video Card ($119.99 @ NCIX US)
Case: Fractal Design Core 1000 USB 3.0 MicroATX Mid Tower Case ($39.98 @ OutletPC)
Power Supply: Corsair CX 600W 80+ Bronze Certified Semi-Modular ATX Power Supply ($44.99 @ Newegg)
Total: $446.93
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-06-15 04:22 EDT-0400