Thanks a lot ;p do you think that with more ram it is possible to get higher hashrate?
No, not necessary ... yet...
Unless somebody creates a way to run higher tc on 4GB systems without increasing lookup-gap then on a 290 more is needed

I'm running 2 4GB and 1 2GB card in one machine that only has 4 GB system RAM. It can be done!
Sure it can - albeit slowly

What cards and what KH/s?
They're running on YACoin, N=14
R7 240 4GB 2.9 KH/sec
XFX 7750 2GB - 2.2 KH/sec (this one doesn't overclock well)
you can compare them here to determine if they are "slow"
http://yacoinwiki.tk/index.php/Mining_Hardware_ComparisonI'm not sure why you think it can only be done slowly. 4GB is a fine amount for a machine with only a couple of miners, or a number of miners with low memory. I feel I am pushing the limit of what can be done with 4 GB though, and if I ever wanted to expand, I would choose 8 GB.