i wont talk about the relative merits of having a good CPU in games because i tend to go on a bit.
what i will say is that at the higher resolutions the CPU comes much more into play (both CPU and GPU have increased workload but as the resolution increases you will find that the requirments of the CPUincrease much faster than those of the GPU)
my advice is to leave what you have got (other than the RAM you have already ordered) and see if the game works anyway, you might be surprised.
in the past i have got "the sims" and "heros of might and magic" running on a 133MHz P1 and (i think)8MB PCI graphics card
so yeah, keep it up and good luck