DreamSlave2112

I have an HP Pavilion 751n with a 1.8 Ghz Pentium 4, 256 MB RAM with no after-market hardware upgrades. The OS is Win XP SP2 and I have a SyncMaster 150MP LCD screen (not the best for games, I know)
For e-mail, writing, net surfing, etc, it runs fine. I primarily play two games on it, CivIII Conquests and SimCity 4. CivIII starts to bog down in the later stages of the game, as the AI is processing turns for several computer players. I suppose this is just the nature of the game and only a faster chip would solve that, but I could be wrong.
The main issue is when I play SimCity 4, which any fan of the game knows is HEAVILY graphic intensive. In the bigger cities, zooming in takes awhile and the buildings look blocky at first and take some time to show finer resolution.
If I want to do a light upgrade, what would be the best option, exp. for SimCity 4? Memory, chip or graphics card (which I'll bet is integrated into the motherboard and prob. uses sytem memory). Any particular brands or types?
thanks.