<<Could I change one of the cards for one more powerful eventually, or do I have to do both at the same time?>>
Yes,you can change for more powerful cards later but only with 1 single card or 2 of the same kind,you can't just replace 1 of the 8600gt,it won't work.
<< if I get vista, can I install it without any net access? I haven't got anything where the pc will be placed.>>
Yes,you still can activate Vista by telephone.If you go to the activation screen, there should be an option that says "other ways to activate". From there you can choose to activate by phone and when you tell it your location it will give a list of numbers to use.
<<what do you guys think is the 'life' on this system before I start struggling>>
Sorry,I don't want to be mean but some of your specs are already dated by today's standard and yes you can still play new games at medium to high setting depanding on your monitor's native resolution(you didn't mention what monitor you already got and the package doesn't seem to include one at all)but you'll certainly start "struggling" trying to play on higher resolution such as 1900x1200 on 24" monitor.