Wow, with $450 and you already have a 550Ti? Don't even look at an Athlon 6400! That's like 100 years old in computer years!
You could so something like this:
PCPartPicker part list: http://pcpartpicker.com/p/D3ik
Price breakdown by merchant: http://pcpartpicker.com/p/D3ik/by_merchant/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($129.99 @ Amazon)
Motherboard: MSI 880GMS-E41 (FX) Micro ATX AM3+ Motherboard ($44.99 @ Newegg)
Memory: Kingston Blu 8GB (2 x 4GB) DDR3-1600 Memory ($42.99 @ Amazon)
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive ($59.14 @ Amazon)
Case: Rosewill REDBONE ATX Mid Tower Case ($36.00 @ Newegg)
Power Supply: Antec Neo Eco 620W 80 PLUS Certified ATX12V / EPS12V Power Supply ($49.99 @ Newegg)
Optical Drive: Samsung SH-224BB/RSBS DVD/CD Writer ($22.95 @ Amazon)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($91.17 @ Amazon)
Total: $477.22 (- another $20 off power supply not shown)
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-02-12 11:23 EST-0500)
fx-6300 is about 3-4x as fast as the Athlon 6400+.