I just purchased the Intel Core 2 Duo 6400 (2.13 GHZ) for a new computer I'm building.
 
I'm building a $1400 gaming rig.
 
Come to find out the minimun system requirments (CPU) for the game I want to play is 2.4 GHZ.
 
Its an MMORPG (Vanguard)  Does this mean I wont be able to play it without lag?
 
I also have  Radeon X1950PRO 256MB   ( http://www.newegg.com/Product/Prod [...] 6814102061 )
 
and 2 sticks of G.SKILL 1GB 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) System Memory  
http://www.newegg.com/Product/Prod [...] 6820231085
 
Should I exchange the 6400 for a 2.4 GHZ when I get it, or do you think I will make do with no lag?  I dont know anything about tweaking...but maybe that would be an option too?
 
Would appreciate any and all advice.

Since it doesn't specify the model, I would say you should be good to go.
 
The 2 Ghz Core 2 Duo is more than a little bit better than a 2.4 Ghz P4.  What they SHOULD have said is 2.4 Ghz P4 or better.  That would cover any Core 2 Duo as well.
 
The processor you bought is one of the best on the market, it should be no problem at all.

wtf is vangaurd?? didn't that come out like last year??? lol u can make do with a 800$ system.