just throwing this out there:
played this on a dual 2.26GHz intel (P8400) with 4GB DDR2-800 and a GF 9700M GT. it ran awfully.
when i built a system with a 3.0GHz quad amd (P2-940BE) with 8GB DDR2-800 and a GF GTX 260 C216, the game flies now.
it's hard to say which is the bigger upgrade since i boosted both so tremendously. i do know hardocp ran a very interesting article comparing various games' performance numbers on different CPUs while using 2x 4870X2 in CFX (to limit graphics bottlenecking). they compared a couple intel quads to the amd tri-core and quad core. in gta4 in particular, the tri-core was still distinctly below the performance of all the quads.
conversely, the quads all scaled together fairly equally. architecture didn't seem to affect the issue so much as number of cores, and the base speed of the cores. aka, 3.6GHz core i7 vs. 3.6GHz core 2 quad vs. 3.6GHz phenom2 x4 all scaled fairly equally, but a 3.6GHz phenom2 x3 fell behind.
it also didn't seem to be the cpu being inundated trying to fill the cards with data, because gta4 only supports 2x GPUs. so no tri-sli or quad or cfx or whatnot.
http://enthusiast.hardocp.com/article.html?art=MTY0NCwxLCxoZW50aHVzaWFzdA==
as to something like the Q8400 vs. the E8500, yes, the E8500 would likely be better for the majority of other current titles that are only really capable of dual-threading. ideally, get the best of both worlds: get a quad with a good multiplier on it, and OC it to 3.6GHz+
in the next 5 years, i think it's highly likely we'll start seeing a lot more multi-threading scaling to at least 4 threads if not the 6 of the x360 or 7-8 of the ps3, and eventually N threads. as the 7th gen consoles pick up steam, developers are starting to get the hang of multithreaded code.
FSX and GTA4 and SupCom are the only titles i know for sure take full advantage of a quad core. more will surely come though. depends what you want your upgrade cycle length to be...