No - RAM in this system will not help much. The lagging is likely a result of poor graphics card(that you did not list) and only 2 cores running relatively slow. Save your money for a mobo/RAM/GPU upgrade.
-Bruce
You should be able to pick up a Q8xxx or Q9xxx quad core cpu cheaply on ebay for example. If you can get some extra 1GB DDR2 memory cheaply then that would help as well.