Yeah, that Celeron is gonna kill you. Im running an older Athlon FX-57 at about 2.9GHz (man, that thing used to be awesome... ;( ) and I can get 3 screens running without too much difficulty. Im not sure I'd want to go inside a city though. Adding anther 2g of RAM will certainly help, but I dont know that it will be enough of a difference.

Your best and cheapest bet would be to replace mobo/CPU + RAM.

Good luck!