[Build Help] trying to build a budget gaming pc!!

i have a graphic card or video card whatchumaycallit that i bought few years ago but never used which i'm trying to build my pc around but it's been really long time since i looked into computer parts and i'm a little lost on what i need to buy. the biggest concern was what motherboard will fit this. i also never use AMD processors before and are they any different than intel? should i get AMD phenom II or the Intel i5? or something even cheaper? and how do i know what fan will NOT block my slots on the motherboard so everything fits like the ram, hardrive and graphic card. is it trial and error? is it also good idea to get a mini-tower? it would save room around my desk area so that's good but is it not ideal? i know the graphic card i mention is NOT the greatest and latest so that's why i'm having a bit of trouble figuring out what's enough or not enough power.

my budget for this pc is around 400-600 dollars if possible. can someone make some part suggestions that are good for a decent gaming pc... i'm going to only use this for games and entertainment. thank you guys!!