I'm undecided, the Intel has more cache and the AMD has more Bus speed and I don't know very much about computers. Here are the full specs:

Intel Core 2 Quad Q9400 Processor BX80580Q9400 - 2.66GHz, 6MB Cache, 1333MHz FSB, Yorkfield, Quad-Core, Retail, Socket 775, Processor with Fan

	Specifications   Processor Speed:  Q9400 / 2.66GHz   Processor Interface:  Socket 775   Processor Class:  Core 2 Quad   Processor Core:  Yorkfield   Cache Size:  6MB L2   Bus Speed:  1333MHz   Additional Technologies:  Intel® EM64T   Enhanced Intel Speedstep   Execute Disable Bit   Intel® Virtualization   SSE   SSE2   SSE3   Quad-Core   Intel® Thermal Monitor 2   SSE4.1   Architecture:  45 nm   Wattage:  95W   Core Stepping:  R0   Fan:  Included
	AMD Phenom X4 9950 Quad Core Processor HD995ZXAGHBOX - Black Edition, 2.60GHz, 4MB Cache, 2000MHz (4000 MT/s) FSB, Agena, Quad-Core, Retail, Socket AM2+, Processor with Fan   Processor Speed:  9950 / 2.60GHz   Processor Interface:  Socket AM2+   Processor Class:  Phenom X4   Processor Core:  Agena   Cache Size:  2MB L3   4 x 512KB L2   Bus Speed:  2000MHz (4000 MT/s)   Additional Technologies:  MMX   Enhanced Virus Protection   SSE   SSE2   SSE3   Quad-Core   AMD64   AMD Virtualization   Cool'n'Quiet 2.0   HyperTransport 3.0   SSE4a   Architecture:  65 nm   Wattage:  125W   Stepping:  B3   Fan:  Included   And yes I'm trying to build a PC for gaming

Intel ...easily. Use any hardware comparison charts. Intel has been spanking AMD since the inception of the Core 2 Duo architecture.

Price difference for your selected CPU's:

AMD - $184
Intel - $269


I personally suggest the Intel Q6600 for $189. Still beats the AMD CPU, costs about the same and can be overclocked on the stock CPU fan really well.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115017

The Intel chip wins, but like Viper1 said, the AMD chip is much cheaper so you should compare it to others in its price range. Clock speed is not an indicator of performance!

Against the Q6600, which is Intel's offering in the AMD price range, the X4 9950 beats the Q6600 in games in most cases:

http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3344&p=15

This shows the X4 9950 leading in 3 of 4 games.

So, get the AMD chip unless the Q6600 is much cheaper at the time. If you can afford the Q9400, then get that as AMD has no competition in that range.

It depends on what you're getting as graphic card aswell... if you're getting an ATI card, I'd go with the phenom. If you're getting anything else, and price didn't matter, then go with the intel brain.

I'd like to point out something though. The phenom is the only real quad core processor out there even though for some things (look for it on anandtech) the Intel C2Q out powers it.

I would also go with the intel chip since I just put one in my machine It has been running flawlessly.

Thanks guys! I'll go with the Q9400 then, I'm thinking about getting an ATI Radeon 4870 PCI-Express with 1GB memory. Will I be able to play games with this processor and hardware for at least 5 years? I'll be also getting 4GB of RAM...

Most certainly.

Yes. That's a great video card choice. You are using a 64-bit OS, right? Otherwise the extra RAM will be pointless.

yup I'll be using Vista 64-bit, How much RAM can it handle? Maybe I'll upgrade to 8GB in the future...