Get the 4 GB, not the 2 GB. As others have said, you'll need it soon, if not already. You'll also be able to turn texture compression off and get better looking scenery/aircraft.
12 GB RAM tri-channel, 24 if you can afford the added expense.
Keep the viz at 10. No more than 15. Visibility is an FPS killer.
Turn the cars OFF. It's a FLIGHT simulator, not an autobahn simulator. Cars eat up CPU resources you'll need to feed the GPU...a lot!
Get Windows 7 and run XP in Boot Camp. nVidia drivers for OpenGl are terrible in OS X. You will get double the FPS in Windows, all other things being equal.
Turn down the shadow detail. Not a big hit, but some.
Do NOT use AI aircraft. It's buggy, silly, and drains resources.
FPS in XP increases proportionally with CPU clock speed within the same generation of CPU. Flash your 4.1 to 5.1 and swap the CPU for a W3680 or 3690. You can run faster RAM then too!
Go for the SSD. Don't listen to those who say it doesn't matter because it does. Loading is faster and it will reduce, if not eliminate, pauses for scenery loading, which can occur even with load scenery in background checked.
Do all of this and you can crank textures and detail, AA/AS filtering to the max. You can also boost cloud puffs to around 25% without too much of a hit.
I did all of the above with a 4.1 to 5.1 flash, W3690 and GTX 670 SC 4 GB. I can get 45 to 50 FPS at KSEA on the ground, 70 in the air with the Baron during chase, more on the panel. I've hit 100 FPS elsewhere with roads etc. replaced by very high resolution photoscenery and still use some custom objects. Again, be careful cranking up weather (cloud detail/layers), visibility, and object/tree density. No cars, birds, balloons and other needless CPU hogs. I'd also stay away from HDR...not worth it. Your single biggest problem with any Mac Pro running XP is keeping a high end GPU fed fast enough with old CPU architecture. I'm still chasing the holy grail of maxing out the important, relevant settings (for flight simulation), and those for scene quality, with never less than 70 FPS. I'm close, but not there yet.