Although clocked at the same speeds as a HD 5870, the 5770 has half as many stream processor and texture units. The memory bus is also 128-bit, as opposed to the 256-bit bus in the 5870. Subsequently, the card tends to be a little slower than the older HD 4870 in some games, and the two cost about the same amount of money. Nevertheless, the 5770 has the advantage of DirectX 11 support, as well as a couple of ATI's other seldom used newer features, such as Eyefinity. The 5770 is a good mid-range card, but spending a little more on an HD 6850 will give you a considerable performance boost.
It took a while for Nvidia to get 400 series cards on the market that hit the price range most consumers are interested in, but the GTS 450 is just such a card. The 450 has only one Graphics Processing Cluster where the GTX 460 has two, but the 450 is clocked faster to compensate for this a little. It benchmarks quite close to Radeon HD 5770 and HD 5750 GPUs, and it is priced competitively, so it's definitely worth a look if you want an affordable upgrade to DirectX 11.
One notch down from HD 5770 sits the HD 5750, with the key difference being fewer texture and shader units. They need less power than a 5770 but they still have a single 6-pin power connector on them. With 1 GB of GDDR5 RAM they are quite capable gaming cards at average resolutions. The HD 5750 is likely due for some price cuts now that the Radeon HD 6000 cards are available, so I would expect this chipset to come close to the $100 mark before long, if they don't release an even cheaper 6000 card first.
ATI has been making every effort to dominate low-end cards as well as high-end ones, and the HD 5670 is a good example of this. It has around half the processing power of a HD 5770 and it is intended to supplant the 4670 in ATI's line-up. The 5670 comes with up to 1 GB of DDR5, and benchmarking quite close to a GeForce 9800 GT in most games, it performs respectably for a card in the $100 price range.
Determined to offer an HD 5000 card for every segment of the market, Radeon has also released the half-size HD 5570. This card is not a gaming powerhouse; it delivers only slightly better performance than the older HD 4600s, but it's good enough get by on a typical 19" monitor. It's one way to enjoy some PC games on a tight budget, and it will fit into even the smallest entertainment system cases.