There aren't many models,on G86 it's just 8500/8600GT/GTS,I don't think there will be a 8600ULTRA most likely it will be a 8900GS similar to 7900GS but it will take a while until is launched.
512GDDR is not a necessety for a card like 8600GTS,to use the full power of the 512GDDR you need to play the games at 1600+ with 2-4XAA/16AF HQ Textures at those settings the card will choke more than Eminem on his first performance.
Also the price will most likely not be 249$ more like 219$ or lower because at 250$ you will buy a 8800GTS that has 96 SP 320/320 and although it dosen't use 80nm and GDDR4 it will clock to 1800Mhz on mem and at those speed's it will most likely beat 8600GTS and by a noticeable margin.