I've been told, as a general rule of thumb, that you can expect to get about twice the performance for a given set of otherwise equal hardware developing for a fixed platform.
I suspect that most of the improvements to GTA4 relative to the console version will be a result of the significantly greater amount of both system and graphics memory on even mid-range systems, though PC system memory is significantly slower than that on the consoles. PC graphics memory throughput is of course many times faster than the consoles'.
It will be interesting to see how it turns out. Naturally, they could also totally stuff the conversion, which wouldn't help anything.
Either way, I'm really looking forward to it. I have never even seen it on the consoles.