My Mum's been complaining that her fairly new Dell C521 PC is quite slow, which is understandable as it's running Vista Home Premium on a 20" LCD, but with only integrated graphics and 1GB of RAM (was a bargain PC though!). Therefore, I'm looking at adding more RAM to it when I'm up there at Xmas. It's got 2x512MB DDR PC5300 already, would it be best to add a single 1GB DIMM, or 2x512MB? Reason I ask is because I've heard of four sticks causing timing problems etc, but at the same time I don't want to lose dual channel because it'd probably cause the graphics performance to suffer (which when running Aero, isn't a good thing). The CPU is an Athlon X2 4000+, if that changes things. Thanks for any help.