Only the bill in the Senate is proposing to increase the potential amount past $8K -- to $15K, but of important note, the $15K proposal is not "refundable" -- it would only be to the amount of your tax liability (but could be split evenly over two years).
Whether any of these bills eventually pass is unknown. All have been referred to Committee -- where most bills die.
That there is enough interest in Washington to have five bills in the loop though bodes pretty well. My personal opinion is the $8K credit will be extended. I'd be surprised if the $15K passes as it's exactly what was in the original Stimulus bill that the House "corrected" to $8K.
But I could be completely wrong. We're talking about politicians after all...