Then close email apps. If game running well then nothing to worry. Background process can make a cpu slower, like Celeron and if many program open, then ram will be full. So you cant switch to another apps or file quickly. Closing unnecessary apps to free some memory and cpu to speed up your pc, if you dont want to update your hardware.
Minecraft is a java based game, so it will utilized your cpu fully. So dont worry.
If you want to add another memory module then you have to use same frequency as you are using now. If you use high speed ram than old memory module will not run. If you want to update then G41 DDR3 supporter mobo will be cheap. But you have to buy a new cpu too, cause your old celeron 336 is not supported in new mobo. But if you are not happy with your system, then updating to a new budget system will be great.
512mb ram is good for you if you are not using multitasking frequently and close unused apps. I am still using 512mb ram in my old pc for web. It is faster for me.