IMHO...
No way in HELL you'll be able to run the game with 2gb ram, in satisfatory conditions. It's a pain even to run SH3 with 2gb ram, using vista. Actually, to be honest, forget running any 2009/2010 game in vista with 2gb ram, its just a headache.
And the videocard performance matters more than VRAM, speaking in terms of graphical options you'll want to enable ingame. In another words: a 1gb 9600gt will run the game crappier than a 512mb 250gts.
Based on what I've seen from other games, from what SH3 and SH4 uses from computer performance, and from those screens, I would go for 6gb ram if you're using Vista/7. I'm not even sure if SH5 will support XP, probably not. And most likely, SH5 can be purelly DX10.
Of course, ubisoft can choose to make such option ranges enough for you to run SH5 with a 7800gt, 1gb ram and a simple dual core processor, but when it comes to run it all maxed out, in a 1680x1050 (which most people use nowdays), in a very satisfatory framerate, Im sure the nvidia 9 series wont manage to do it, neither 2gb ram.
If you think about the jump on system requirements we had from SH3 to SH4, it was huge. And don't forget, SH3-SH4 interval was shorter then SH4-SH5. Based on today's avaliable hardware and the pictures/videos from SH5, I'd say its much likely they'll push the limits of nowdays hardware, when running the game with all graphical options maxed out.
But, too soon for worring about changing rigs. Let's just wait for the game to be released, and possibly try to run it on what we have right now, so we can have a better idea on what to buy, what not to buy. Plus, by doing that later on, we'll have better hardware for a lower price.
