i wouldnt go bulldozer. for gaming terms it goes i5 2500k > i3 2100 > phenom II X4 > bulldozer FX-8150
(bulldozer and the phenom II X4 are about equal, but i put the phenom ahead because of the cost)
price wise it really depends what your after. if you want a gaming rig that will happily play the majority of games at middling settings your looking at £400 for the case and the bits inside, but if you want PC capable of maxing out every game out there with Nvidia 3D surround your looking closer to £2500 for the case + internals, and more if you want watercooling with it.
if you give us a budget, and what parts your looking to buy we can give you a hand.
if its a lower end budget (and your current PC isnt incredibly old) then it would also help if you tell us what your current parts are because then we could tell you if theres anything worth recycling for the budget your looking at. This can be done with a combination of a program called CPU-Z and a 30 second look inside of your PC.
info from CPU-z:
processor, motherboard and graphics card make and model
ram type (DDR1, 2 or 3) and size
info from looking inside the case:
if your DVD drive and hard drive are sata (small flat cable) or IDE (massive ribbon cable)
power supply make + model
case make/model