I agree with Reather. You will be able to play all games smooth with one 7900gt. Graphics cards are getting better and better all the time. You are better off buying a Very Good single card instead of two Mediocre cards. You would also need a SLI capable Motherboard for two cards, and a more powerful PSU.
Also, in the next 6 months or so, DX10 cards will be out. Thats when you will want to upgrade.
The processor is your choice. Either will suffice for gaming and usual tasks. You may be correct in upgrading later: Due to Intels new chips, there may be a decrease in the price of AMD processors in the next month or 2.