What is your total budget?
Both i7 920 and Q9550 are around the same price, but the i7 setup will cost about $200 - $300 more for a total build.
That's not correct. An X48 board will cost around 249~279, except on some EOL or open box ones. The X58 costs around the same price. On the other hand, DDR3 kit's price is coming down to be within $150. Therefore the greatest price disparity between them is most likely less than $100.
With that said, unless you have some LGA775 parts you want to keep, X58 is mostly a much more logical choice.