Good CPU (processor

For Price to Performance wise I'd get an AM3 processor, by AMD. Has DDR3 so it's future proof. AMD Phenom II compared to i7 the AMD price tag is a lot cheaper and really really good performance wise. Personally I would get a

AMD Phenom II 720 BE for $119.99
http://www.newegg.com/Product/Prod [...] 6819103649 (I'd use that for most gaming)

Or if you use a ton of Applications while your game go with the
AMD Phenom II 945 $179.99
http://www.newegg.com/Product/Prod [...] 20II%20945

Both those processor will last you a very long time, no need for an upgrade. Also do you plan to Over Clock your processor? If so make sure to get the BE (black edition) models for the unlocked multiplier.

But if you want to go full blast and get the best of the best! Get an i7 from intel price ranges from $279.99 to $999.99 with $569.99 in the middle.

For my new rig I'm just making I decided to go with the AMD Phenom II 945 since you can't beat the performance for the price. I won't upgrade my processor for about 2-3 years as it stands. 3 years at max 2 years minimum.

Also recommend getting an after market cooler for Over Clocking.

For gaming get an AMD.

No Intel is better all around. But for gaming AMD will give you much more bang for the buck.

my budget is around $100-$400can

Is that your budget for the processor or for the whole system? If the processor then you can get just about anything you want, but I'd still go with the 955 if purely for gaming. Save the money for the video card.