Well a good way to learn about PC's, is to understand what you've already got... the bill of lading/part sheet that came with the PC is a good place to start. Like a lot of things, it takes a few hours of research and reading reviews to figure out what's a good fit for your budget. I follow the PC BBS thread I linked below, and would've copied and pasted this there...
I'd recommend a video card that has at least 3GB of RAM, just to keep those textures at the ready. If you dig around on that thread, you can find other builds listed, along with some handy sites like: http://pcpartpicker.com/list/
VicariousE
Guess you could browse around or ask here: http://www.newgrounds.com/bbs/topic/982338/82
They sell DIY PC kits online, but more often than not, one or two components likely won't have the kind of quality you're looking for... if the PC runs for a day straight with no problems, it's a good sign.
Have a look at Newegg or TigerDirect, maybe go for an i5 kinda setup, since there's tons of motherboards to choose from. Also, how old is your current rig/OS? Might be able to re-use parts and software for the new PC
FuShark
Thanks Vic, this is some helpful advice.
I'm not 100% sure on how old the parts are. It's a full sized tower so I'm sure the case will be fine and all the fans are in working order. Graphics card isn't up to par anymore and my biggest issue is that the motherboard can't support newer ram. The OS is Windows 8.1