do you have one of these A6/A8/A10 AMD processors? There may be a problem because these processors share the same memory bus for the cpu and the graphics adapter (which is fine for most applications).
You do not need a expensive graphic adapter for lr at the moment because LR does not use the gpu for rendering. I have a AMD HD6450 with a 1920x1200 pixel display without any issues.
Lots of brushing is known to slow down Lightroom. A faster CPU is probably the only way to speed Lightroom up for this particular problem.
The alternative is to use Photoshop or Photoshop Elements to do the brushing.
A graphics card will not help.
As dj_paige said - if you do lotsa detailed brushing, it's gonna slow Lr way down - even a swanky new turbo-powered cpu will only help so much..
If you really need to brush the heck out of it, then you need to either do it using a pixel-editing app like Photoshop, or find a way to consolidate destructively (bake brush-strokes into the image data) from time to time in Lightroom.
ProxyEditor (a free plugin I wrote) was designed to allow you to do the latter - you brush on a consolidated RGB file, and changes are retrofitted to the original source image. Note: it will still take a long time to export the original image, but at least editing will be fluid (Lr will remain responsive).