We always hear so much about ram, cpu upgrades and video cards to increase fs preformance but not much about sound cards, how would a new top of the line sound card really affect flight simulator?

Performance wise verry little if any at all. slight gain going from onboard to slotted card (in reducing CPU cycles so those can goto FS you dont notice that even). Also with the X-FI ram found on the Fatality cards you wont notice a great perfomance leap in fs, FS9 will still be a CPU hog, upping that will give the greatest noticeable performance gain