Gpu for multiple hd video playing and editing

You need more video card processing power. I'd check out an ATI 5770, which ive seen as low as $130ish after rebates on newegg.com

I would personaly go with an Nvidia GPU. Adobe supports Nvidia's cuda video Acceleration thingi.
basically, you take the stress from the CPU and put it on the GPU.
I would recomend something like a 200 series GPU from Nvidia, maybe a GT240 1gb GDDR5, they are runing for $100 in newegg.
I would not recomend a 400 series GPU because i read somewhere that they don't official support it just yet.
Hope this helps