Depends what you mean by Intel's quad core... There's Intel's Core 2 Quad and then there's the i5 and i7 (i7 being hyperthreaded, so 8 logical cores on 4 actual cores). The AMD Phenom II X4 is probably around the Core 2 Quad, but i5/i7s are better.

The way I see it, i5/i7's the way to go, but if you're on a budget you cannot beat Phenom II X4.

Simple version is that any Athlon II or Phenom II under about $180 is faster than its similarly priced Intel counterpart.

In terms of performance (but the AMD one is cheaper so better value):

Athlon II X3 425 = Intel E6300
Athlon II X4 620 = Intel Q8200
Phenom II X3 720 = Intel E8400
Phenom II X4 925 = Intel Q8300
Phenom II X4 945 = Intel Q9400
Phenom II X4 955 = Intel Q9500
Phenom II X4 965 = Intel Q9550

Most modern PC games (with a few exceptions, like ArmA II and GTA4) run just fine on a decent dual-core CPU, provided that you have a good enough graphics card. The Athlon II X3 435 is the best "price:performance" value in CPUs right now for a gaming PC; if you absolutely must play one of the few CPU-bottlenecked games out there, however, grab a Core-i5 (and, in the case of ArmA II, a SSD.) But most of you out there will only need the 435.

Strictly for gaming, you wouldn't need much more than a dual-core processor. Namely the AMD Phenom II X2 550 black edition. It runs at a cool 3.10 Ghz (3.8-4 Ghz overclocked) and tops most quad-cores in many games (like Crysis).

My advise would be to go for the Phenom II X3 720 BE. It has similar performance to the X2 550, plus a bit of future proofing as well.

Unless you have the cash to burn, that is. Then the expensive i7's and i9's are at your disposal :)

The 720 is no longer on AMD's price list, has climbed in price recently, and has largely been replaced by the X4 630 (at $114) or X4 925 (at ~$140) There's also a 2.9GHz X3 435 at $87.

Tri-cores improve game framerates more than you think.

IME, more never hurts when it comes to CPU. I honestly never advise people who seriously want to game, to cut any corners on their CPU purchase just to save a few $$. It comes down to personal preference and need of course, but from what I have seen over the years - people who spend the extra few $$ where it counts on things like CPU and video card usually seem to end up happier in the long run than those who dont.

Just one minor example was GTA4 launch on PC... up until that point many had been saying that dual core was enough or even better due to higher clock speed... with GTA4, there was much wailing and gnashing of teeth so to speak amongst folks who did not have at least a tri-core and wanted to play GTA4. Games that make good use of two threads will benifit from more cores, especially if you set thread affinity to cores that are not being used by system and background processes.

 Its true that quad is not a total necessity, but I know I personally would not go back to fewer cores willingly. Quad is worth it in many, and often subtle ways.

Except more DOES hurt - when it comes to your wallet. Especially when you'll never, ever use that extra processing power for anything if the most CPU-intensive thing you do is game. Also, GTA4 on PC is a poorly-ported mess and is the exception rather than the rule.

Lots of other games are starting to use more threads, though. And moreover, DX11 games are supposed to be able to use more threads better as well.

If you're building a computer today, unless you're on a really tight budget, there's no use not shelling out a few dollars more just to get a quad, really.

Can you name these "lots of other games," please? I just took a quick look at the games released on Steam in the past couple of months, as well as the upcoming games for the next month, and only one (Dark Void) has a quad-core processor listed under its "recommended" specs. All of the rest list dual-core.

Also, I wouldn't hedge my bets on DX11 quite yet. It'll probably be at least another year until games start to come out that use it properly, and two before DX11-required games start to pop up. By then you'll more than likely be able to buy the equivalent of a Core-i7/Radeon 4890 setup for a pittance.