Building a computer for $500 is very cheap, and it will be very tight.
I would recommend you just stick with your current processor and not upgrade, it's a 3rd-generation i5 and is still viable in today's standards.
I think your system will only ever pull about 400W, but I'll include a 450W+ for possibility to upgrade the GPU or CPU later on.
Considering now that you have the GPU, CPU, MOBO and RAM, we can focus on building the other components with a now, decent budget.
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
Storage: Western Digital RE3 1TB 3.5" 7200RPM Internal Hard Drive ($55.00 @ Amazon)
Case: NZXT Source 210 Window ATX Mid Tower Case ($50.98 @ Newegg)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($57.99 @ NCIX US)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Wireless Network Adapter: TP-Link TL-WN725N 802.11b/g/n USB 2.0 Wi-Fi Adapter ($8.88 @ Amazon)
Total: $262.83
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-10 16:04 EDT-0400)
This includes a great Hard Drive, Power Supply and has the Windows 8 you wanted.
You still have lots of money though, to upgrade your other components. Since your i5 currently is still a great processor, we should be looking to put the extra money into a nice graphics card. I know that you already have your 750ti, but for the money you've saved you can invest in a GPU which is much stronger than it.
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($27.99 @ NCIX US)
Storage: Western Digital RE3 1TB 3.5" 7200RPM Internal Hard Drive ($55.00 @ Amazon)
Video Card: Gigabyte Radeon R9 270X 2GB WINDFORCE Video Card ($194.99 @ NCIX US)
Case: NZXT Source 210 Window ATX Mid Tower Case ($50.98 @ Newegg)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($57.99 @ NCIX US)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Wireless Network Adapter: TP-Link TL-WN725N 802.11b/g/n USB 2.0 Wi-Fi Adapter ($8.88 @ Amazon)
Total: $485.81
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-10 16:07 EDT-0400)
In this build I've added a R9 270X, which is a much stronger card than the 750ti, and also a CPU cooler.