I went from using DDR2 to what I have now, I never noticed a big difference in DDR2 667, 800 OR 1066. I don't know if you will notice a big difference in speeds.

Over clockers and people like me who use programs to compare each others rigs notice. Going from 3gb to 6gb you may notice as things seem to load a little quicker.

You may want to shop around for Ram as the price listed for the 1600 is a little high in my opinion.

http://www.newegg.com/Product/Product.aspx?Item=N82E16820145224&cm_re=PC3-12800-_-20-145-224-_-Product

i mostly play online games or i set my gfx to the max. i just noticed it lagging a little. in fallout new vegas if i turn my character to fast it has like a 1-2 second pause. will ram fix that or should i keep the 3 gigs and go for a new gfx card? i have sli gtx 260. was looking at gtx 460 since it has dx11 and i wanna play games in dx11

A new GFX card would help. I would get 6gb memory , you don't need any more than 6 for gaming.  I have not owned a Nvidia since the 7900 series. You might want to look @ the newer ones - 560 , it may be a bit better ? The memory u had selected will work fine.

If your still under warranty- save the stuff you replaced, I understand they will only accept troubleshooting results from the original config. So put the GFX card and ram in the desk

i got the 6 gig dominator 1600hz. i dont notice a huge improvement so i might return them