What specs do you have?
If your Mac mini is running OS 10.7 or later: from the Apple menu, select About This Mac. Click More Info bar. Click Storage. Then use the window which appears to check how much hard drive space is available on your iMac mini.
If your Mac mini is running Mac OS 10.6 or earlier: Switch to the Finder, open a new Finder window. From the sidebar, select your hard disk by clicking it once. From the File menu, select Get Info. The window will show the disk space available on your iMac's hard drive.
To find out how much memory (RAM) your Mac mini has, go to the Apple menu (top left of your screen) and select About this Mac. You’ll be given a screen that displays your processor speed and how much RAM is installed.
To find the video card installed in your Mac mini, click on the Apple symbol at the top left of the Mac menu select About This Mac (the first entry), click on the More Info button at the bottom of the dialog. This brings up System Profiler. Look under the Contents column on the left and find Graphics/Displays under the Hardware section. Click on this, and the details on your graphics subsystem, including vendor, chipset and ROM version, will appear on the right. System Profiler can also be launched from the Utilities folder under Applications.