I'm not the world's leading expert on processor design, but from what I can tell from the schematics and die pictures I've seen of the Westmere 6-Core CPU's, it only *looks* like they stuck 2 3-core processors on the same die. Everything I have read about Nehalem (and now this current die-shrink) has said how modular and scalable it has been designed to be; Intel saw the limitations of their early dual-core designs in that they *weren't* particularly modular or scalable, and I can't imagine that they would repeat that mistake.
That said, from what I can tell, they laid it out like they did (so that it looks like three cores on one side and three on the other) in order to minimize the maximum distance(s) between certain key components (they stuck the queue and most of the uncore in the middle). It is my assumption that although there appears to be two L3 caches on the die, that any core can directly access any of the cache. I base this mostly on my understanding of how the QPI system works on a DP platform -- one link is for communicating with the Northbridge (Current DP NB is tylersburg 5520 while SP boards usually use X58) while the other is for direct communication with the other processor --- I would assume that if processor 0 can get data directly from the cache on processor 1, that any core on processor 1 can access any data on it's internal shared cache.
Your argument seems to be "GTA 4 runs like shit on my dual core CPU, and they say that you really should run it with on a quad, so therefore it will run even better on a 6-core CPU."
No offense, but that's a load of crap. GTA 4 runs like shit because it is for all intents and purposes a completely non-optimized console port. I've seen PLENTY of people with computers hands down above and beyond mine (tri/quad SLI with watercooled and heavily overclocked GTX285/295's (respectively), along with watercooled and heavily overclocked extreme-edition i7's) have difficulty running GTA 4 well.
I have a pretty good computer for these days (Core i7 920 CPU on an EVGA E759 X58 motherboard with an EVGA GTX 275 SC {at least that's how it came flashed}), and I have been able to graphically max out EVERY console port I have ever run at a minimum resolution of 1600 x 1200. -- titles like Call of Duty: World at War (I played it completely maxed out at 1600 x 1200 and I think my FPS was capped at 60 (I think I had it set to not do FPS above the monitor's refresh rate), and Mass Effect 2 (which I run completely maxed out at 1920 x 1440 and it has never gone below 60 FPS in any of my 2-3 playthroughs totaling at least three days ingame so far).
Basically, what I'm saying is that if your argument for more processor cores is based on the fact that badly written/optimized code runs better with more processor power, you need to find a better argument.
Statistics are much more enlightening than anecdotal evidence.
"There are three kinds of lies: lies, damned lies, and statistics." -- Mark Twain
Numeric stability is probably not all that important when you're guessing.