The Inspiron 545s only takes low-profile cards. I believe that neither the 450 or 460 are available in low profile. (At www.newegg.com
I see a low-profile 430, which probably isn't a fast card, but it is a Fermi series product.)
It also has a 250W PSU, which is lower than recommended for most add-in cards.
I'm not sure, but I don't think that a standard ATX power will fit in the case.
If you wish to move the motherboard to a standard case, be aware that the connector for the front panel stuff (power switch, power LED, drive LED, etc.) is a Dell proprietary design. That may not make it impossible, but it'd be more work than with an industry standard motherboard.
My advice: treat the PC an an appliance, as Dell intended. If you want a PC that uses industry standard components, build one. (It's what I've done since 1997.)