There are pros and cons to each sim but on the hardware side, the Saitek Yoke is good but has a *very* strong center detente which is unrealistic and you might find it annoying.
The CH Flight Yoke (not the new one with the silly buttons on the front), is good as are their rudder pedals. Saitek sell the throttle quadrant (bundled with their yoke) separately and it's worth a purchase to use with a yoke (you can buy a couple and clip them together for a cheap multi-engine throttle quadrant). (The throttle quadrant bundled with the saitek yoke connects via a DIN plug rather than USB which the individual one does which is more flexible).
You *need* a TrackIR - honestly, once you've used one, you'll never go back.
PC spec wise, if you're going FSX, you'd be looking at nothing less than an Intel i5 (one of the new ones if possible) or i7, with a decent graphics card such as an ATI 58** series or Nvidia equivalent. Likely cost about £700-800 for a reasonable rig.
If you're going multi-monitor, ATI has the advantage with their eyefinity cards which will support 3 monitors for widescreen gaming. Nvidia and other cards would need a Matrox TripleHead2Go which are limited in choices of resolution and monitor.
FSX pros are that most new add-ons are for FSX, and the FS user base is now 75% on FSX and 25% still on FS9.
FS9 (Flight Sim 2004) will run much better than FSX on a comparable machine - it's a matter of preference which you prefer, and if you want to run some of the newer add-on scenery/wx and aircraft.
X-Plane is a bit of a mixed bag. It has FS9 frame rates and runs quite well, but the included aircraft are shockingly bad and the add-on market is small in comparison with Microsoft. FSX is where it's at if you want heavy metal or complex procedural aircraft.
However, there are some amazing aircraft around for X-Plane such as the Carenado Mooney which knock the spots of the FS equivalent. Also you get sloping/non-level runways in X-Plane.
X-Plane needs a bit of tweaking and has a steeper learning curve than FS9/X, but once properly configured can perform better in certain regards, but it is limited in other regards.
If you want a decent fidelity set-up, then FSX is the way to go with REX2 as a scenery/weather add-on, or ActiveSky X for weather.
Microsoft have a new simulator in the works called "Flight" but it'll be a few months before more details come out on that.
From the JustFlight forums, this is a reasonable build for getting decent FSX performance:
• £98 Asus P8H67-M EVO Intel H67 Express Socket 1155 Motherboard
• £184 Intel i5 2500K Retail
• £75 Corsair Memory XMS3 8GB DDR3 1333 Mhz (2x4GB)
• £197 EVGA GeForce GTX 470 Superclocked NVIDIA Graphics Card
• £50 Western Digital Caviar Black 640GB WD6401AALS Hard Drive
• £75 Windows 7 Home Premium 64bit OEM
• £80 Corsair Gaming Series GS700 PSU
• £23 Asus TM-B11 Black Micro ATX Tower Case
• £13 Samsung DVD Writer, SH-S223C/BEBE, SATA, Black, OEM
Total: £795
That should give you an idea.
To be honest, get a decent spec machine, then just go get all 3 sims and see which one you prefer. You'll get FSX Deluxe/Gold for £30, FS2004 for less and X-Plane for around the same. Try them all out and see which one suits you the best.
(Also check out Lock On:Flaming Cliffs 2 and DCS:Black Shark for the best combat flight sim, and awesome helicopter sim if you're into that side of it!).
Cheers,
S.