You really need an entirely new system if you plan on running any modern titles. The Pentium 4 CPUs all fall below the minimum system requirements for most games these days, dual core is the minimum, and quad core is recommended on most titles. As said above you can throw in another gigabyte of RAM and a 9800GT and that would offer okay performance in games that don't need a dual core, though the vast majority of these are from 2007 and earlier.
i have intel p4 3.0 processor with 1 gb ram . which game graphic card suits my system
If you are in it for gaming, then yes a total system upgrade is necessary.
But if you don't game, and only use your pc for internet / typing letters / watching 1080p youtube and such.
Then probably a Nvidia 8400 or Radeon HD 2400 is all you need.
A typical 8800/9800gt even second hand costs around $50-70 usd and higher for the 1GB versions which are getting to be rare. A 9800gtx/gts250 is a better value for the price. I wouldn't use a G92 card on a p4 anyway. A p4 can't even handle a single x1900xt anyway. Second you didn't say what type of p4 for all I know it could be an 478 with ddr1 meaning agp. Rebuild is your best option and gives you more and better choices about upgrades.