Because the Corsair is much better quality. For example, the 12V rail on that one that you linked to only has 14A, while the Corsair has 30A (the 12V rail is what powers the CPU, GPU, and hard drives on most modern computers, so it matters). Also, I wouldn't be surprised if the one you linked to is rated for 450W maximum, not continuous, and would actually burn out if you tried to run it at that load for any length of time, while the Corsair is one I would trust to deliver full power all day long. Don't go cheap on the PSU - when they fail, they can take out the entire rest of your system.
Yes, im afraid your system wont even run a 4670, which is the best video card without extra power. Most HP Slimlines have 160w power supplies and ive read where 8500gt's have fried from power failure. They usually go with a HD 4550 or less for ugrading, which still is pushing it.