Well, the only help I can be is provide these two links, again, not a direct comparison.
Essentailly, the closest competitor in Anandtech's review is a 7900GS, using the second link you can compare a 7900GS to a 7800 GTX (256 or 512)
Anandtech 8600GT Followup Reviewhttp://www.anandtech.com/video/showdoc.aspx?i=2975
Tom's VGA Charts 2007http://www23.tomshardware.com/graphics.html
Also, would help to know if this was the original 256MB 7800 GTX or the 512 version...512 had a slightly different core and was generally more powerful.
Also, would need to know what resolution you play at. From what I can tell, even at 1024x768 an 8600 GT still doesn't measure up to a 7800 GTX, but at higher resolutions, it will fall off dramatically due to the 128-bit memory bus.
Oh, and one last thing... your title mentions the 8600 GTS, but your body text says 8600 GT. While there is not a major difference between the two, the GTS is more powerful.
Good luck!