okay so first the GTS 250 even though its older, is a much higher tier GPU so it will be the better card. The next thing is that it cant be two PCI slots, it has to be PCI-E, pci express, not sure if thats what you went, and yes you can do it but you will run into some problems with games and drivers, and being its such a low end card it would hardly benefit
No you cannot crossfire those two cards, one is amd, which uses crossfire technology, and the other is nvidia, which uses sli technology (if those cards even support it to begin with). The GTS still scores better because its an actual gaming card, though low end. The 5450 is a home theatre card. Gaming with amd on that series begins with the 5670.