This is a thread best handled by Nick.
My best advice, and what I'm currently doing, is to save up and get an i7 system.
Nick has posted what is required to be happy in FSX, anything less than that list is a compromise IMO, and leads to finding more ways of spending money to achieve what could have been prevented by just buying the right stuff the first time around.
I tried the budget route myself, thinking I could just upgrade in the future, what I wound up with is a killer 'FS9' system that can 'run' FSX. Had I spent another $500-$600 (a lot I know, considering I built my whole system for $650) I could have had a Sysem that would 'play' FSX.
Point is, the economy sucks, and prices of things are coming down in order to increase customer spending, save for the good stuff, and you'll be buying it sooner than you think.
Otherwise, you'll be back here in a few months looking for the same advice.