4870 or 5770 is better if there were no dx11.. i am confused about the differenc

The HD4870 and HD5770 have basically equal performance on current drivers. The memory bus is only one of many factors that go into a video card's performance and is essentially pointless to consider on its own. Besides gaining DX11 the HD5770 also has the advantage of MUCH greater power efficiency. At idle the the HD4870 uses nearly 60w more. If you leave your computer on all the time like I do that translates into a few dollars a month on your power bill.

i am not clear about the difference of bits yet  

+1 @jyjjy.

ATI 4870 (stock)
Chipset Manufacturer: ATI
Core Clock: 750MHz
Stream Processors: 800 Stream Processing Units
Effective Memory Clock: 900MHz (3.6Gbps)
256 Bit Interface

ATI 5770 (stock)
Chipset Manufacturer: ATI
Core Clock: 850MHz
Stream Processors: 800 Stream Processing Units
Effective Memory Clock: 1200MHz (4.8Gbps)
128 Bit Interface

The 4870 may have a higher memory bandwidth, but has slower Core and Memory Clock speeds. All of these specs, (including newer GPU architecture), factor in on the overall performance of a video card... not just the bandwidth.

hmmm i got a little bit in my head.. in short clock speed is more imporetant rather than looking at bits?

All of the factors are important. Each on its own doesn't really matter. Basically you should be looking at performance benchmarks not technical stats. In this case the two cards will give you equal performance but all of the other factors favor the HD5770 to a strong degree.