Final questions before buying my new computer system.

[Freddie]

GeForce 8600GT - That's the best i can get for my budget.

In case you don't know, Newegg has 8800GT-512's starting at 99.99 after rebate. (I realize you may not be in the US.) [Edit: I just noticed you're in Sweden, sorry.]

Combined both RAM and Video-RAM I will have 3328 mb totally...

I don't know how you arrived at that number but I would look at it this way.

(1) With 32-bit Windows and 3 GB ram, you'll have 3072 MB of usable ram.

(2) With 32-bit XP and 4GB ram, you'll have approximately 3582 KB of ram (that's the visible amount on the PC I'm using, which has a 512 MB video card). That's an increase of 510 MB over (1).

(3) With 64-bit Windows and 4 GB ram, you'll have 4096 MB of usable ram -- an increase of 512 MB over (2).

What do you prefer, Vista or XP? (Hardcore XP fan myself)

XP because I'm used to it and it's less hassle. (I"ve got both XP and VIsta machines side by side on my desk.) But if I were buying a new PC today I'd get 64 bit Vista.

If you decide to go with 32-bit, I would get 4 GB of ram because approximately 510 MB of the fourth MB will be usable, and that last 510 MB will cost you only $16.50 (assuming you're in the US), because Newegg is selling 4 GB of 800 MHz brand-name (Kingston) ram for $66.

Can I run 32-bit application's on a 64-bit machine?

Yep.