For straight up line art I recommend InkScape.
I use both programs on a regular basis and InkScape is closer to Illustrator in layout and uses less memory. Synfig is closer to flash, but can be a lil... finicky... if you don't know what your doing. (crashes are common) I only use Synfig for animations and BGs. InkScape is more forgiving if you're tracing over your line art and easier to fix mistakes.
InkScape can also be imported into GIMP or PS for fine tuning and fx. Synfig you can fine tune and fx within the program.
Any other questions just shoot me a PM.