Best Single Slot GPU?

[mysticviperx]

My computer has served me well for years, but its time for another upgrade and I'm thinking graphics card. But, after buying a nvidia 550GTX and discovering I can't install a double slot GPU on my mother board, I find my self at a loss as to what card I should get. I currently have a BFG 9500GT 1gb (http://www.amazon.com/BFG-Tech-NVIDI...pr_product_top)

So my question to whom ever is reading this; What is the best card I can get for my computer?
It has to be:
Single slot
1gb
I prefer nvidia, but for the right price I guess ATI will do
It will be used for mostly gaming, with some video transcoding on the side
I have 11 inches from the back to the hard drive case BARELY
I would like to stay below $150 including tax

The more options the better, pro's and con's would be nice.

Scott

ps: please don't let this hinder any responses, but if at all possible I would like to buy it at directron.com, as it is where I purchased the 550GTX and I will be returning it there, (I don't have to pay shipping due to living near there warehouse, I can simply go there and buy it in person) not that I have any thing against any other retailers.