I have a hp pavilion 521 c and want to upgrade it. i want to almost strip all the parts in the computer but i am wondering what the from factor for the case is or how to find the form factor so that i don't get the wrong size motherboard.
http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph07371&tmp_task=prodinfoCategory&cc=us&dlc=en&lc=en&product=77585#N839. A micro atx motherboard should fit. You'll also want to change the power supply and hardrive on such an old system. I would check newegg's shellshocker specials daily if you can order from them. Most include: case, power supply, cpu, heatsink, ram, and motherboard.
This system http://www.officedepot.com/a/products/687478/Acer-AX1430-UD30P-Desktop-Computer-With/ will be $239.99 on black friday. You can't upgrade that cheaply. I recommend it if you have the funds.