I use realityXP's garmin 530 simulator. It's kind of a resource hog because it uses Garmin's own simulator as a platform (which is a 16-bit program), but it's much more realistic than the Garmin simulator included in FS2004. I wish that they had a 430 simulator, but I'll take what I can get.
The best add on by far for any flight sim program is a second monitor. I've had two monitors for years now and I'm here to tell you, you can't go back. I can drag the garmin sim, radio stack, etc. on to the second monitor.
Another cool add on gauge is the sandel SN3308. It's an EFIS/HSI/RMI, etc. guage designed to fit as a replacement for GA DGs or HSIs. LMNOP....
Info
Demo
Just remember, it's only a game.
Dave