The Card
The smaller 40nm design of the Radeon HD 4770 allows it to operate at 750MHz which is the same core frequency of the more expensive Radeon HD 4870. The core configuration features 640 SPUs (128x5) along with 32 TAUs (Texture Address Units) and 16 ROPs (Rasterization Operator Units). All in all, these are extraordinary specifications for a graphics card that costs just $100.
Interestingly, this is a similar core configuration to that of the Radeon HD 4830, however where the Radeon HD 4830 gets its 640 SPUs from a 160x4 configuration, the 4770 does things on a 128x5 configuration. Unfortunately we cannot determine what kind of impact, if any, this has on performance because these graphics cards use different memory bus widths. The Radeon HD 4830 utilizes a 256-bit memory bus, while the Radeon HD 4770 is regrettably crippled by a 128-bit wide memory bus.
Unlike the Radeon HD 4830 and 4850 graphics cards, the Radeon HD 4770 makes use of GDDR5. The memory works at 800MHz (effectively 3.2GHz) on this particular model which should make for an impressive memory bandwidth. However, due to the 128-bit memory bus, the card is limited to a bandwidth of 51.2GB/s which is why it slots in between the Radeon HD 4830 and 4670 in terms of bandwidth performance.
The ATI reference sample board came with Qimonda ICs on board (IDGV51-05A1F1C-40X parts). These GDDR5 modules are rated for 1GHz (4GHz) operation so it should be safe to assume these overclock rather well.
Speaking of memory, according to the ATI specification the Radeon HD 4770 will only be available in 512MB flavors. Given the price and performance target of this product 512MB of memory makes perfect sense. That said, brace yourself for the 1GB versions that will inevitably turn up.
Granted that many of today's games can make good use of a 1GB frame buffer, the Radeon HD 4770 will not have the horse power to run them at the required quality settings, making the extra memory unusable.
Although the Radeon HD 4770 has a thermal design power (TDP) of just 80 watts, it employs a dual slot cooler. This is a rather interesting choice as we would have expected a single slot cooler from this 40nm graphics card. The dual slot cooler provides improved cooling without generating excessive noise we suspect, though in low-end systems that often tend to use the mATX form factor space can be tight, so this should be a consideration to make. Moving on to performance...