Things to check for:
1: Motherboard, does it have a pci-e slot?
2: Power Supply, most retailers like DELL use weak 300w psu's to run most of their desktop models.
3: Processor, if the cpu is too weak(a dual core below 2.6GHz) you may not get the full performance a 450 can give.
4: Monitor, If you're using a resolution like 1280x720, a weaker card would be a better option.
5: Size, if you have a small sized case, you may have problems with non htpc ready cards.