5770 is better in performace wise because 5770 has
Model Radeon HD 5770 Radeon HD 6790
Manufacturer ATi ATi
Year October 13, 2009 April 2011
Code Name Juniper XT Barts LE
Fab Process 40 nm 40 nm
Bus PCIe 2.1 x16 PCIe 2.1 x16
Memory 1024 MB 1024 MB
Core Speed 850 MHz 840 MHz
Shader Speed N/A MHz (N/A) MHz
Memory Speed 1200 MHz 1050 MHz
Unified Shaders 800(160x5) 800
Texture Mapping Units 40 40
Render Output Units 16 16
Bus Type GDDR5 GDDR5
Bus Width 128-bit 256-bit
DirectX Version DirectX 11 DirectX 11
OpenGL Version OpenGL 3.2 OpenGL 4.1
Power (Max TDP) 108 watts 150 watts
Shader Model 5.0 5.0
Bandwidth 76800 MB/sec 134400 MB/sec
Texel Rate 34000 Mtexels/sec 33600 Mtexels/sec
Pixel Rate 13600 Mpixels/sec 13440 Mpixels/sec
Memory Bandwidth: Memory bandwidth is the largest amount of information (in units of MB per second) that can be moved across the external memory interface in one second. The number is worked out by multiplying the interface width by the speed of its memory. If it uses DDR type RAM, it should be multiplied by 2 once again. If DDR5, multiply by ANOTHER 2x. The higher the memory bandwidth, the better the card will be in general. It especially helps with AA, HDR and high resolutions.
Texel Rate: Texel rate is the maximum texture map elements (texels) that are processed per second. This figure is worked out by multiplying the total number of texture units by the core clock speed of the chip. The better the texel rate, the better the graphics card will be at handling texture filtering (anisotropic filtering - AF). It is measured in millions of texels processed in a second.
Pixel Rate: Pixel rate is the maximum amount of pixels the graphics card could possibly record to the local memory in one second - measured in millions of pixels per second. Pixel rate is worked out by multiplying the amount of Render Output Units by the the core speed of the card. ROPs (Raster Operations Pipelines - also called Render Output Units) are responsible for drawing the pixels (image) on the screen. The actual pixel fill rate is also dependant on many other factors, especially the memory bandwidth - the lower the bandwidth is, the lower the ability to reach the maximum fill rate.
a 6790 is definitely faster than 5770, but tdp-wise and price-wise hd6850 or even nvidia gtx460 1gb should be better than 6790.i don't know how much those card are outside us, tho'.
don't go for cards like hd 4xxx unless you really really want to go that way.
what resolution will op be playing at?