Need help choosing parts for my budget PC

[Blisslen]

I've been looking into getting into a better PC, but budget has been a huge constraint for me. After seeing people building gaming PC's off of low budgets to try to beat consoles in affordability, and I'd like to get myself something similar. To start with I'll be grabbing my 6GB of DDR3 ram from my current PC, as well as it's 500w PSU and its 1TB and 150GB hard drives, so that's where I'd like to start from. This PC, to function optimally for me, I'd want it to be able to maintain a stable FPS above all else, as well as be able to run CAD software well (That's what I'm going to school for). My current budget is going to be $400, and that won't include OS or anything since that's already on the hard drives I'm using. Any part suggestions are appreciated, I don't need full builds from anyone who has a suggestion.

EDIT: I'd also like to be able to upgrade this PC in the future, so I won't have to start from scratch when my PC is too old to run the new games and things.

 

[bimbaws]

You need mainboard, cpu, gpu right? So i if you'll work with graphical software better buy nVIDIA gpu card - support PhysX and is better to stuff like that. and from nvidia, GTX is best choice in your budget (GT series has low performance, Titan / Quadro has high prize). then mainboard... there are more questions to solve : what cpu (AMD / intel), must have port for your gpu (pci express 3.0), if you have dual channel ram, you need at least 2 dimm ports, if you have 2 hard drives you need 2 ports SATA (most popular ports for hdds). for mainboard : i recommend asus or gigabyte. now cpu... amds have more cores, but intels are stronger... so this is harder question.. but amds are cheaper, and in some cases, have higher performance.