I always keep open a ton of tabs in Chrome and possibly you use a lot of VMs like @OliverSalzburg
Then you can benefit from tons of RAM
you should at least put your OS on a SSD
So you need a case to put everything in
Then you pick a CPU that matches your budget
In your case, going with an Ivy Bridge i5 should be enough, somewhere between 70-120$
Then you pick a motherboard, depending on certain features you want (mostly based on ports, like USB) go with a brand you know/like