Hey Everybody,
I've got the same 2.53 unibody 4GB MacBook Pro. I'm using Vista Home Premium 64-bit, and under the advice of another thread, I started using Driver Detective a little while ago, really nifty program to take the hassle out of the driver-hunting experience.
My GTA IV runs great (not quite up to par with a gaming computer, but pretty good), but before upgrading from 32 to 64-bit, it was aweful, very laggy. Half of it probably has a lot to do with the GPU underclocking, and the other half with out-dated drivers from the boot camp installer.
Anyways, I tracked the most up-to-date NVIDIA driver back to its website, the latest supporting 9600M GT (Feb. 11 '09 release). It's supposedly in beta, but I haven't had any problems, and GTA is working great.
Here's some graphics updates, if anybody's interested:
XP 32-bit:
http://www.nvidia.com/object/geforcenotebook_winxp_179.48beta.html
XP 64-bit:
http://www.nvidia.com/object/geforcenotebook_winxp64_179.48beta.html
Vista 32-bit:
http://www.nvidia.com/object/geforcenotebook_winvista_179.48beta.html
Vista 64-bit:
http://www.nvidia.com/object/geforcenotebook_winvista64_179.48beta.html
There are a couple platforms supported off the support site, but there you have it. Give it a whirl. In any case, the biggest difference I found was going to 64-bit, although you should still think of upgrading your graphics.
Ciao
Hamish