actually its not the first number that shows how the graphics card performs. the first digit is the series number and the next numbers are actually the indicator of performance. Nvidia launch a newer series and in every series they launch some high performance card, some medicore performance card and some low end performance cards.For example 9500gt the first digit is "9" that means it is a geforce 9series card and generally is manufactured with the same die as other 9series cards(not all necessarily). the next digit "5" is actually the identifier of the performance. 2/3/4/5 is entry level cards or performer. they are just used for running pc and running old games like commandos3, maxpayne-1 etc. running newer games than that is a disaster with those cards they may run it but in low settings and even after low settings they will choke in games like assasin creed or lost planet etc. if u want some gaming performance u need to look for "8"(the 2nd digit) that means 8800gt or 9800gt. But if u want to go very low dont have much money now to spend but want good performance and in smaller resolution like 1024 or 1280 then at least look for "6" in the 2nd digit and in the first digit "9" that means 9600gt. i will not recommand 8600gt because in some games it chokes but really if u cant stress ur budget anymore and need a card now still 8600gt is not really a very bad choice it will run most of the games in lower resolution and lower settings. And u may wonder what about gt or gts. the general hirerchy is GTX> GTS>GT>GS>LE.
so the summary is u need to look at the 2nd digit to know what kind of performance it can provide. and u cant compare 8600gt with 8800gt. it will be make more sense to compare 8600gt with 7600gt with 6600gt - becoz all have the "6" in the second digit and means "medicore" performace" so u compare medicore cards with the same level of cards. so 8800gt will be compared with 9800gt. i hope this will clear why 8800gt is better than 9500gt.