Quick Settings is a great app, and it will tell you how much space you have on your SD card. You can also just look under settings/sd card and phone storage. You can set it to have an icon in the notification bar which sound like it would be handy for you if you need to check it often.
I also use an widget called mini info which gives me my battery level, sd level, and internal memory level. You can then tap it for more information. Keep in mind that the sd and internal levels will not update "live", so a recent change may cause the readout to be inaccurate for a little bit.