just to make sure you know, The GTS 450 does not come in a 256 bit interface.
GTS 450 - 128bit/1gb GDDR5
GTX 460 - 192bit/768mb GDDR5
GTX 460 - 256bit/1gb GDDR5
I just put an MSI GTX 460 in my pc and I love it. Great quality.
If we are going to help you figure out what gpu would be the best for your system we need to know: budget, use (like gaming or other) and psu make and wattage.