However, the error message you are getting may not be from your SD card being full.
The Moment (and all Android phones) have a limited amount of ROM to install applications. You mentioned that you like do download and install apps. If you have more than 40 or 50 apps installed from the market, you may be pushing the limits of the free ROM in your phone.
If you'd like please post the exact error message you're getting here. It could help others to troubleshoot the same problem.