Nitro,
Your CPU is likely the biggest bottleneck in your current rig. You can confirm this by monitoring your CPU usage (Task Manager / performance tab) and GPU usage (I use MSI Afterburner which may also work to monitor GPU on non-MSI graphics cards).
My vote would be save your cash and put it towards a new motherboard, cpu, and RAM when you have enough. Good Intel i7 options would range from around $600 to $1000 and would completely blow away your Athlon so far as performance for Premiere Pro CS5/5.5/6 goes.
Regards,
Jim