m7ncg 400 memory question

Ok I have two 512mb sticks on the system now but I would like to increase it to 2 gigs total. Can I add a 1gig stick and have everything run smooth or would I be better off with 2 ,1 gig memory sticks?
Thanks in advance for the help, Mark

Another thing if this helps I am NOT overclocking anything on the pc at all and I do not plan on it in the future. I having trouble running some games smoothly with a lot of backround programs running. Some of theese unfortunately I cannot shut down to gain more (free) memory. Thanks again, Mark