I'm not sure if you've been following
this X-Plane 10 thread, but it's gr8 to see X-Plane 10 maturing.
BUT.... only once it is 64 bits (will be when 10.20 gets public) this sim can be used to its full potential.
Most "very capable" Macs with lots of scenery, aircraft and plugins installed hit the 32 bits RAM limit too easily.
Once the app gets close to 3 GB RAM consumed, it will crash, annoyingly informing you of "getting out of memory".
This is the first game (sim...) I have encountered that actually NEEDS 64 bits!
I have spent a fortune on the game itself, huge amount of payware scenery, payware aircraft, plugins and iOS apps which complement X-Plane.
Most "high detailed" scenery (payware KLGA, payware EHAM payware KLAX, etc.) are a no-go area if you set the grfx pretty high. Not even "highest", but simply "pretty high".
I use a Mac Pro '08, 8 x 2.8 GHz, 8 GB RAM with Radeon 5870 for flying X-Plane, and even this relative old beast gets the app crying for its mother when it's going to load the above mentioned scenery. The settings are no way maxxed out (VRAM about 650 MB out of the 1 GB available), and the computer can handle the FPS easily.
It really is so annoying when your hardware is easily capable of handling higher and more detailed settings, but that the app itself cannnot handle the RAM needed.
In short:
X-Plane 10 is gr8... just keep the settings low enough....once it's 64 bits, it will be superb. Then get hold of the best hardware you can afford, then it will be perfect.