The Phenom II and the new FX CPUs competes against Intel's older Core 2 Duo / Quad CPUs. Intel's 1st generation (Nehalem) and 2nd generation (Sandy Bridge) Core i3/i5/i7 are faster than their AMD counterpart. In games the slowest Sandy Bridge dual core Core i3 CPU can compete very well against quad core Phenom II X4, that are clocked at higher speeds, in games.
FX is not much of an improvement over Phenom II when it comes to games. It is basically a new CPU that performs the same as the Phenom II.
if you can fit a phenom ii x4 in that motherboard, get one before stocks run out. list of phenom ii at newegg.com
rumor has it amd is eol'ing ph ii (probably except zosma) and athlon to make room for newer cpus and apus. i noticed that newegg hasn't restocked their ph ii 955s yet.
edit: don't listen to rumors, make your decisions based on facts. if your motherboard supports ph ii, make sure they're available from your place of purchase in case you decide to buy one.
if you want to build a pc faster than your current one and if you want to build now and you have the budget, you could get one with the following components:
core i5 2500k/2600/2600k
coolermaster hyper 212+ $34
asus/gigabyte z68 motherboard (may be one with pcie 3.0 capability) around $140-170
8 gb ddr3 1600 ram
90-120 gb ssd
amd radeon hd 6950 / nvidia gtx 560 ti/570
you could also wait a bit longer (q2-q3 2012) and build yourself an ivb system with new 28 nm gfx cards.
the i5 build will certainly be faster. can't say about ivb until it comes out.
you don't need intel to build a fast pc. you can build a reasonably fast pc from many choices available without intel cpus.
In my knowledge the fastest CPU in the present market is the quad core CPU. It is advertised with speed of 2.4 or 2.66GHZ. An old P4 chip can get 3.0GHz. Is there anything faster than 3.0GHZ?
I am not sure about its range, but I think it will be expensive.