CPU is now irrelevant
FPU is now irrelevant
GPU is fast becoming irrelevant
ASIC mining possible
CloudHashing possible
You need to do more research on mining before you purchase a PC to mine with. If you would like to mine Bitcoin you shouldn't start in the past. As more powerful hardware and hashing power comes online, the difficulty of finding a reward is ever increasing. This is a technology and hardware race to continue earning.
So, if you are intent on mining come up to speed on ASIC Mining. From Bitcoin Wiki:
An application-specific integrated circuit, or ASIC, is a microchip designed and manufactured for a very specific purpose. ASICs designed for Bitcoin mining were first released in 2013. For the amount of power they consume, they are vastly faster than all previous technologies and already has made GPU mining financially unwise in some countries and setups. https://en.bitcoin.it/wiki/Mining#ASIC_Mining
Most ASIC manufacturers have a pretty big backlog of orders, and often by the time you finally receive the hardware it could be obsolete. Read company and community forums to get a feel for who you should trust.
For example - Butterfly Labs is just now shipping preorders from March and April for it's products. In the meantime other ASIC providers hit the market and dropped hardware prices.
You may also want to check into Cloud Hashing. This is just a fancy term for hosted hashing. Essentially, they host and manage the hardware, power, internet, etc. You rent hashing power in Ghashes. Pay attention to the fees with the plans. I read one plan where you paid for 100 Ghs, but 50% went to maintain site and hardware. There is no real standard at this time, so read carefully.