The requirements have not yet been set, but they should be less than those for Supreme Commander.

For Vista I would recommend a minimum of 1GB of RAM (as opposed to the 512MB).

There will be no differences between 2gb and 4gb ram in games.
Of course, if you´re using a programm, which needs mass ram etc. but for games are 2gb enough.

Hum, I'm not sure to clearly understand what you mean but there are already games that use more than 1gb of ram by themselves.

When Supcom first came out, on the biggest maps (which mind you are huge, 5-8 minutes for a fast flying scout plane to fly from one corner to the other), the game could use over 2GB of ram. However, it would be a game versus 7 AI with something like 800-1000 units for each side (up to 8000 units overall). Since I don't think Demigod will get anywhere near 8000 units, not to mention that the maps should be smaller, then this issue shouldn't be a problem.

To help find out how your game will run, download the Supreme Commander Demo. Please note though, that this demo was never optimised, so Demigod should definitely run smoother than this.

When I have played the story part the demo my fps dropped below 10 shortly after I have started my game. However when I was playing the skirmish map I had always until the end of the game 30-40 fps and in the main game you will have the same speed.

i have played some time ago a 81x81 km map in Supreme Commander Forged Alliance with 4 people, it was a 2v2 match. After 15 minutes the game paused itself (not froze) and didn't continue. Then I have looked in my task manager and I could see that Supreme Commander took 1,5 gb ram and the rest was taken by Windows Vista. I am able to play every single 2v2 game on 40x40 km fine until the end, even if it lasts 1-2 hours.

I am playing on a notebook with the following specifications:
2x2.5 ghz
geforce 8600m gt
2 gb ram
tweaked vista

On my big desktop computer the game didn't run very well as soon the maps reached 40x40 km with 2000 units.
1x2.4 ghz
Geforce 7800 GTX
1 gb ram
usual xp (no tweaks)

I can recommend you a dual core processor strongly.