AT least you know it's the CPU holding you back. Other people have come asking what's wrong with their new 8800GT and when we finally dig out their system specs, it's their old CPU holding it back. Worse yet is when they are on another CPU socket and have little to no CPU upgrade options only to end up feeling they wasted all their money on the video card.
$100 is a good budget for an AM2 CPU and will put you well up in the X2 5000 series. (X2 5400+, 5600+, 5800+) It's a good idea to check with MSI and see what CPU's are supported on your specific motherboard revision before buying one. Also, specific CPU's may require you to flash the bios to be properly recognized.
Wow, that sucks.
This morning the 5000+ BE showed out of stock.
Now it shows as a deactivated item
In that case, as pauldh suggests, any of the X2 5000 series will give you the best performance for your $.