with my motherboard if my old 6600 gt was compatible? i dont know what motherboard i have. yes? no?
No, you cannot make that assumption. The 8600 is a PCI-E card while the 6600GT comes in both PCI-E and AGP. You will need to know what kind of graphics card controller your motherboard has, PCI-E x16 or AGP 8x/4x.
You can find out what motherboard model you have by bringing up the Start > Run box and typing "dxdiag" without the quotes and hit Enter.