Can I get a USB graphics card of 1 GB or more and will it increase my gaming experience? I am using G42 HP notebook. Please give me some suggestions?
No it will not.
I'm going to vote your answer anyway, as its clearly the most correct, but a little explanation would have been good too!
USB graphics cards don't add any graphics performance. They are primarily designed to add graphic outputs to connect additional monitors. In fact it will use some of your CPUs performance further limiting your gaming experience.
The solutions I know of are ExpressCard expansion cards or Thunderbolt to PCI-Express.
One of them is the ASUS XG which requires an ExpressCard slot and ~at this time ~ is quite aged and doesn't really offer the performance for current games.
The other one is the Sonnet Thunderbolt to PCIe expansion chassis.
The third solution I found is DYI
I didn't see either interface Thunderbolt or ExpressCard on the G42 so I'm afraid there is no solution for you.
How to Upgrade Your Notebook Graphics Card Using DIY ViDOCK
it depends if G42 HP notebook (BIOS) support external graphic card, check als if your laptop support Mobile PCI Express Module
An external GPU will reduce CPU load if the software is designed to use an external GPU.
ViDock 2: are external graphics cards finally here? http://optimitza.cat/news/2009/07/01/vidock-2-are-external-graphics-cards-finally-here/#sthash.n97IayH9.dpuf