Microsoft isn't stupid, they won't leave out people with current computers.
There was probably two market plans for fsX.
1: Make it work on current computers perfectly fine, and get more sales of the game.
2: Make the game require Windows Vista and DX10, causing slightly more Vista sales.
No.1 is definatly the best choice.
You wasted your cash on the GFX card...I don't think it's DX10 compatible...

True, however a 7900GT is a very fast card and will run fsX on fairly high settings. Infact, E3 is on a 7800GTX.
Congradulations!
You have just volunteered yourself to become the UNofficial beta tester of FSX when it first comes out.

If the developers at M$ are correct about the general system requirements then your new rig should handle it. But if it doesn't then I don't know what rig could.
Beta tester? I have had most Microsoft games and they've never been buggy. If you want to become beta tester get a game called Big Rigs: Over the road racing.