Most IMPT single piece of hardware for FSX performance

What matters most to FSX performance? (If the budget permits the purchase of just one (b)leading edge item and filling the box with otherwise mid-range stuff, where do you splurge?Amt. of RAM?Processor?Video Card memory/performance?And how do you compare clock speeds between P4's and dual core chips? The P4 #'s seems higher, but it's hard to imagine a 3 year old design is "faster"