Alabalcho :
USB interface (and video adapters inparticular) tend to be "slow". If you expect to run games on that secondary monitor, it will have poor refresh rates.
Any $100 PCI-Express video adapter will perform better than your on-board video. Just make sure it will fit inside.
# yeah I'm willing to pay £60 or so for an external graphics card possibly and just buy a second 21in monitor if you think this would work better, i would need one with HDMI output, but then I'm thinking i could just use my other computer that PC that should be able to handle two monitors any way, i read some ware that any computer with that run windows 8 can run two monitors, but my other computer has a dedicated graphics card anyway so it has a bit more power also Samsung offers some software to support multiple displays, what do you think.
Plus I have to consider available power from my factory stock PSU, because if i had to replace my PSU and add a graphics card it would cost £100-£200 quid considering my computer only cost like £350, .... But like i said earlier i could use a second monitor for my other computer.