Better saving for a nice light with the right colour spectrum and proper reflector since most of the light you are using now is probably wasted. What is the wattage on the bulbs? C02 is always a good thing but if you dont have the light to utilize it properly you're probably wasting your money.
With decent light, 1.5Watts per gallon and up you can probably see good results from C02 but then you will also have to start fertilizing. It really depends on the plants you have and how much you want them to grow and really look. Some plants demand high light, others just need low.