Well there is always the 6450 which isnt much of an improvement, in my opinion the best options in the low profile range would be the SAPPHIRE HD 6670 1GB GDDR5 Low Profile, which has pretty decent performance, but personally i would wait for the Sapphire 7750 Low Profile cards which would have better performance, however I have no idea how long you will have to wait, Iv seen a few articles on it but no release date as of yet.
Hope this helps.