Depends how much you want to invest with flight sims. You could get FS2004 and a joystick and learn plenty for not much money.
Or you could go yoke, pedals, radio stacks, VFR photoscenery, aircraft addons, head tracking, high end machine running FSX. You'll want some advice then though on getting the best out of it all from one of the sim forums.
I would judge what you 'need' on how much use you'll get from it. Personally I have pretty 'all singing' sim, it's great for when I don't get to fly for real as much as I would like. I've done a lot of VFR stuff on it too with the photoscenery so it's more than just for instrument understanding.
Remember too you don't need to 'play' a sim. You can use it as a tool in anyway you like, repeating one part over and over, stopping it, moving the plane. Stopping and looking at instrument readings then considering what it should have looked like, etc etc. In a way the more you use it like this the less you need all the accessories as you are not so much 'playing' it as using it as a learning aid. If you want to build up to flying an entire flight as you would for real, then the more 'toys' you have the more realistic it gets.