What kind of games do you want to play? And at what resolution?
I say skip the 8600GT and get a 9600GSO for not that much more if you can afford it.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121251
As for your power supply, you have to look at it. There should be a label on the side of it that tells you how many watts it puts out and how many amps are on the 12V rail (important for graphics cards).
Nice card, i defiantly can afford 10 extra bucks for the increase, but when it says under memory type-GDDR3 does this have anything to do with the mobo? or the RAM?