Have you unscrewed the plate where the card meets the back of the PC case? There may be a latch instead of a screw at the top of the blanking plate that needs to be released.
There's also possibly a latch at the point where the card plugs into the motherboard, at the end of the slot furthest from the back of the case, which you may have to press to release the card.
There's a useful site
here to compare benchmarks of video cards. As you can see, the AMD Radeon HD 6450 is comprehensively outperformed by newer cards.
If you are going to get a new card, set a budget and then see what the best card you can buy is (at least according to the benchmarks).
SD
[edit] Just loked up the service manual for that PC:
http://downloads.dell.com/manuals/al...nual_en-us.pdf and there's a "card retention clamp" at the back of the PC case to hold the expansion cards in place. You can also see the latch at the end of the card slot.