My advice is to learn to use a new Application and get comfortable with it (working reliably) before you even contemplate starting an important project.
FCP X is working very well for a great number of people, but it does have its quirks too.
To be able to help you find out why you are getting crashes, you need to supply a lot more information than you already have - length of projects, what video codec you're working with, Whether your media is optimised, where your media, projects and events are stored - how the HDs are formatted etc etc.
Don't lose heart! It's an awesome application - you just need to see why it's not working well on your system.
Here are some of the most common ways to improve performance (you may have already addressed some of these but it's worth reading them.
FCP X runs very well (not perfectly, by any means) on my 2009 MacPro 2 x 2.26 GHz Quad-Core Intel Xeon with 16 GB RAM and ATI Radeon HD 5870 1024 MB
First, check the spec of your iMac against the system requirements:
http://www.apple.com/finalcutpro/specs/
Particularly your graphics card. If it's listed here, it's not suitable:
http://support.apple.com/kb/HT4664
If you are getting crashes, there is some conflict on the OS. Create a new (admin) user account on your system and use FCP X from there - if it runs a lot better, there's a conflict and a clean install would be recommended.
Other ways to improve performance:
Create optimised media.
Hide Waveforms at all times when you don't need them (both in Browser and Storyline / Timeline). They take up a lot of processor power.
Create folders in the Project library and put any projects you are not working on currently, in there. (Twirl it closed to hide them).
Move your Projects and Events to an external HD (make sure it's formatted OS Extended - with journaled on or off) and run from there.
The biggest single improvement I saw in performance was when I upgraded the RAM from 8 GB to 16.
Andy