Radeon HD 4670
You might have just done a double-take. The Radeon HD 4670's numerical name isn't much higher than the Radeon 4550's. You might also do a double-take if you take a look at the cards, because while the Radeon HD 4670 is a bit larger, it is still a rather small card compared to most of the hardware currently available.
The performance, however, is quite a bit different. The Radeon 4550 is an acceptable gaming card for some games, but it is geared more towards HTPC applications and towards adding the ability to game onto systems that are too small to fit larger video cards. The Radeon 4670, on the other hand, is a competitor towards the 9600GT. Most performance benchmarks indicate that the HD 4670 is, overall, slightly less capable that the 9600GT. But the difference is often less than 10%, which means the HD 4670 provides value, competitive to the 9600GT. This also means that the Radeon HD4670 should be capable of running most games without issue. As little as a year ago, you would have had to pay $200 to get this kind of performance, but the Radeon 4670 usually retails for a mere $70.
When compared against the 9600GT, features will likely be the deciding factor. Although ATI is advancing its own physics and GPGPU technologies, they so far don't have the same kind of support as PhysX and CUDA. If you have an application that makes use of CUDA, or you want to take advantage of PhysX features in games like Mirrors Edge, then the 9600GT is the obvious choice. The Radeon 4670 is the better choice for use in combination with HDTVs, however, as its solution to providing audio and video over HDMI is smoother than the 9600GT's - you simply need to put the card into the PCIe slot, install the drivers, and you're good to go.