Hi,
Being quite an experienced simmer, I thought I'd put my spoke in aswell.
First of all, you have to remember, there is one serious flaw in MS FS2004. They haven't medelled air movement properly, with the result that you can't get downdrafts and updrafts while you are flying. This means that while you flying in any sort of programmed weather, things like windshear and turbulence are nothing like the real thing.
Also, when it comes to the flight dynamics of aircraft on FS, it is better to buy an add on than use a freeware product. Don't get me wrong, most of the freeware available is excellent and very usable. However, with the amount of time you'd need to spend getting everything exactly right with the dynamics, textures, cockpit etc.. you end up having to pay for the product. Aircraft development for FS can take months, even up to a year to get it just right.
I would recommend for a 737 package the PMDG 737 series
www.precisionmanuals.com
And for the Airbus A320, The PSS A320 Pro package is only just wonderful.
www.phoenix-simulation.co.uk
These packages give you a real feel of how these aircraft really handle. You'll find the PMDG737 definitely handles much more responsively than the default 737.
Also, if you're interested, there are other add ons you can get, that will enhance your simming experience. Programs like FSMeteo and Active Sky will download real world, up to date weather, including ALOFT information for flying at high altitudes.
FSNavigator is a moving map display/FMS, that displays all current navaids including low and high airways for navigation. It also has a large database of DP's/STARS for planning into and out of an airport. Although, the aircraft packages I mentioned earlier have their own built in, fully functional FMS.
If you want a multiplayer experience, there are a lot sites out there than can provide that. However, there are a few sites that also provide ATC coverage. The one I'm a member of is
www.vatsim.net . This has a large membership and you can nearly always get ATC coverage somewhere in the world. The US and Europe are the busiest areas and in someplaces, they follow procedures as they do in the real world. The London TMA is a challenging place when its busy on VATSIM and you are expected to follow procedures to the letter!
But, when it comes to the most accurate flight dynamics and phsyical modelling of the air, the only sim that does this properly is X-Plane. Not as flashy as FS, but if want as real as it gets on a computer, X-Plane is the one to get.
Anyway, hope this helps.
Cheers!