i'm a bigtime pc gamer...
Now your pc is more than good enough except for one thing, your onboard vid card. Basically that vid card is either a 64mb or 32mb and than it uses hard drive space to get the rest of the 256mb. In an essance its not a real 256mb vid card because hard disk memory is slower than a 100 year old t-32, lol.
Anyways spend about 100$ and get a half way decent Agp8 or PCI-E x16 card. Thats whats makin your game lag.
Good luck
Have fun.