Assuming a tower configuration with the CPU at the top and the card slots at the bottom:
Inside your PC, measure along the slot the card will use from the rear bracket forward. If there's nothing in the area between that slot and the one below it along that line within 21cm, it will fit.
Now measure from the face of that slot straight out to where the case cover will be when it is closed. If that distance is more than 11.1cm, you're good to go.
If you find these instructions too confusing, cut out a piece of cardboard 21cm long and 11.1cm wide. Set it along the PCIe slot and against the rear slot bracket. If nothing touches it, the GTS450 will fit.