The Gigabyte 970 Mini is it. Definitely the most powerful card that will fit in any case (because it is only the length of the board). It has a maximum TDP of 145w, but you can just get a program like MSI Afterburner or Asus GPU Tweak to permanently set the power target to 90% which will only lose you 5-10% performance, but give you more headroom on power requirements. It uses one 8-pin connector (or 6-pin + 2-pin).
$340
http://www.amazon.com/dp/B00OQUMGM0/?tag=pcpapi-20
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125706
http://www.bhphotovideo.com/c/product/1096665-REG/gigabyte_gv_n970ixoc_4gd_gtx_970_mini_itx.html
http://www.superbiiz.com/detail.php?name=GA-970IXOC&c=CJ
http://www.microcenter.com/product/441042/GeForce_GTX_970_Overclocked_4GB_GDDR5_Video_Card
**Edit- You could also just get a regular 970, I just like the smaller one because it fits any case. The normal sized cards are much longer but can run a few degrees cooler, but if you plan on reducing the power target at all, the temps will probably be cool either way. Other 970:
http://pcpartpicker.com/parts/video-card/#c=186&sort=a8
I would get the Evga ACX 2.0 970 if your case can fit a full sized card. I only recommended the smaller card because your old card is also a small card and because it takes only one 8-pin. The regular sized cards take 2x 6-pin, but the EVGA one comes with adapters that work off molex connectors.