Here is a nice little comparison chart.http://www.tomshardware.com/reviews/best-graphics-card,1987-6.html
Stuff in the Chart is generalized.
The person maintains it says that in general until you get 2-3 levels different, you will not feel a big difference in real world use.
If you note, you 8600GTS is one level higher than the 3860.
As a result, you may have a slightly better card or its a wash.
As Hallow pointed out, the 512mb of memory does not really help cards around that performance level. The reason is they are not powerful enough to run resolutions and color depths that would need that much ram.
The extra RAM is usually just for marketing.