I have a msi rs480m2-il motherboard http://www.pcstats.com/articleview.cfm?articleID=1802 which currently has two 512MB memory modules in it. Can I add two new 1GB memory modules such as http://www.crucial.com/uk/store/mpartspecs.aspx?mtbpoid=DCBC3E8CA5CA7304 ? I am worried that I will be mixing different RAM modules.