Need more focus, it might be PCIe 1X or it might be and older CNR port.
Also is it a recent or older DELL?
I've primarily worked with the Optiplex, Workstations and Laptops, not the dimension line, so is is recent or old?
If it's OLD it's probably CNR, if it's recent might be PCIe 1X.
On Dell's website it actually answers both your questions if this is your rig;http://www.dell.com/us/en/dfb/desktops/dimen_3100/pd.aspx?refid=dimen_3100&cs=28&s=dfb
See what a little Google can do? :P
But if you're looking to upgrade, you're probably better off just buying a new system, PCIe 1X cards are expensive and geared towards 2D not any heavy lifting.