My suggestion, as an addition to Lion_Ts, is not to go over 384MB if you're targeting users with 512MB of RAM. Try to keep it at least 128MB off of the total memory, so you can play nice with the operating system like Windows XP and other programs. In other words, if you are going to target those with 256MB of RAM, go with the nexus of 128MB. Try to go as small as possible, along with your levels. Use less texture sizes as possible as you can. I hope this helps.