A little background first. I have used Microsoft Flight Simulator since its' very beginning, when "as real as it gets" was a series of angled hollow shapes. I can go back further if you want me to, such as a 737 "Simulator" that ran on a ZX81 and you flew from Square A to Square B - if you got it right you saw the "scenery" which was a third Square labelled - er - C.... Enough of this frivolity - Microsoft Flight Simulator has come a long way and can now be viewed as a serious training aid in some respects - I have had an actual licence for 16 years years now and still use FS to refresh such things as instrument navigation.
I also have a pretty decent PC - 3Ghz twin core processor, 4Gb RAM, GTX280 Graphics - in other words, not just a wordprocessor. I have both FS2004 installed which I have used since it came out (in about 2004 I think!) and also Flight Sim X (10). So what are the pros and cons?
FS2004 is smooth stable and reliable. It has plenty of free add-ons available, and is still well served by the aftermarket providers. I can wind this up to full settings and it doesn't flinch. Add ons such as Photographic Scenery have a large margin of performance in which to operate, so the performance remains smooth with frame rates in the order of 50+ (anything over 25 is smooth).
FSX is a great deal more demanding. Functionality is pretty much the same as FS2004, but the graphics are noticeably improved. The glistening of the sun reflecting off the wing of a turning aircraft is magically recreated. As you accelerate down the runway, the graphics make you feel as if you've been pushed back into our seat. There are interior views by the dozen, and wing mounted cameras, etc. There are a host of other improvements over 2004 as well, but nearly all are aimed at pushing the boundaries of sound and vision - or reality if you prefer. Like 2004, FSX is well served by the aftermarket providers and there are apparently limitless add-ons to be sought after. These are excellent buys if you are looking for a continuous source of subsequent Christmas presents!!
BUT, all these improvements in FSX come with a price. Not money - performance. The program is demanding, much more demanding than FS2004, which means that you'll probably be making some compromises and pulling back the graphics sliders. I installed Photorealstic scenery on FSX as I have it on 2004 - my computer just could not cope realistically (it could cope poorly!) and I have now taken it off.
At the end of the day a realistic flight simulator experience requires a number of factors to be present - but right up there at the top is fluidity, smooth flight movement. It doesn't matter how pretty the graphics are if, when you come onto final approach for landing (when the textures that the sim is required to handle are generally at their most complex) the frame rate dips down to 10 or 12 Frames per Second. The illusion is destroyed immediately (and chances are you won't be able to land your aircraft either!).
This could explain why, after all these years with bth FS2204 and FSX on my machine, it is invariably FS2004 that I load up for an evening's "flying". I just find FS2004 is still "as real as it gets".