To play the most intensive of todays games even at 10X7 max without aa and af, you need 7800GT/7900GT or higher cards.
Well, it purely depends on what game you are playing, considering X1600 is or even 7300GT is a Graphics accelerator, you'll probably play any games on it fine. Well, I can certainly vouch for that, simply because even my 9800Pro plays them fine.
1) 7600GT will probably not run FEAR well even at 10X7 with soft shadows.
I agree, but perhaps, you fail to notice, that with Soft-Shadows, even high-end cards stutter too. But, it's a fact, that FEAR is one of the most poorly coded game, right after, Halo, BattleField II etc. And I might also add Serious Sam II in that list as well.
2) Can a 7800GT play it at max at 10X7?
Yes, I am not sure, but you may consider my above reply please.
3) GRAW needs 512mb for high settings, right?
Sir, every game on video cards with higher frame buffer size and decent core will run fine, although, for your consideration, I might add, that it runs fine on my 9800Pro at 800x600 with Medium, so, you could probably play it fine on X1600 or 7600GT or X1800GTO easily.
A X800XL/6800ULTRA/1600Pro level can't handle it at those settings, 7900GT has only 256mb.
You are only considering frame buffer size at the moment, but not how many Pixel processors and Vertex processors a card has.
Your only options: 7800GTX 512mb/X1800XT 512mb/7900GTX/X1900XT/X, the latter may play even at 12X10, so the former maybe required for 10X7.
And rather expensive...