Couple of comments for ya:
These other guys are right. There are a few areas in your system that you just can't skimp on -
1) effects. go with 24 bit effects or you will eventually get frustrated.
2) monitors. If you cant afford real ones, buy a good set of headphones and use those as monitors. Not the greatest solution, but a damn site better than "ZIM GAR" monitors.
3) patience. I've sort of developed a proceedure for mixing where I do a mix, listen to it for a few days in the car, living room, friend's house, and daughter's walkman. I write down the corrections to that mix but do nothing and allow at least 2 days to pass before I mix it again incorporation the changes I've written down. Repeat the process until I'm happy with the product - which may wind up taking a few weeks depending on the project.