Need advice on improving my FPS on Vindictus and other games

I have been playing this Pc game "Vindictus" released by Nexon. And have been playing for over a year. I have been encountering a problem with my fps, it's under 30. Even though I set my settings to the lowest, it still doesn't seem to be improving.
Here are some system information:
OS: Microsoft Windows XP
Manufacturer: Gateway
System Model: 838GM
Processor: Intel (R) Pentium (R) 4 CPU 3.00GHz(x86 Family 15 Model 4 Stepping 3 GenuineIntel)
RAM: 2.50GB
Graphics/Video Card: Nvidia Geforce 8400 GS
Please share any advice on this, before I go to Memory Express, recommended by the players of the game. I want to get a clear idea, what the problem may be. And if I need to upgrade or buy a new pc.
 
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