Parallels works very well. I have not encountered any issues as yet. You will need a minimum of 16GB RAM to run a Windows based charting software on Parallels. This is so that you an allocate 8 GB to Parallels while being able to work on your Mac applications at the same time. I also suggest purchasing a Mac Book Pro laptop so that you can be mobile.
Alternatively, if you’re restricted by budget, you might consider purchasing a Windows based laptop that you can dedicate to trading. I would consider 16GB for this laptop too. You can’t have too much RAM!
If you want to continue using your iMac then consider using Boot Camp so that you can boot into Windows or OS X. You won’t be able to access both Windows and OS X at the same time however. Make sure that the version of Windows that you want to run will work on your iMac.