The 9600 GT is the fastest low profile card of which I am aware and here is a decent one at a good price:
ZOTAC ZT-96TES3L-FSL GeForce 9600 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready Low Profile Video Card - Retail - $79.99 AR
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500103
Here is the chart showing 4670 (the slower 4650 was not on the charts but I beleive it is about 2/3 the speed of the 4670) vs. 9600 GT:
http://www.tomshardware.com/charts/gaming-graphics-charts-2009/compare,1195.html?prod%5B2472%5D=on&prod%5B2457%5D=on