http://secure.newegg.com/Shopping/ShoppingCart.aspx?Submit=viewThis includes Monitor and Keyboard, if you do not need these then disregard those parts.
In case that doesn't work (sometimes the cart will work), here is an image to just show what prices you could be looking at.
I built a machine with these parts for my sister recently and she can run Portal/Portal 2 and Sims3 (only things she plays) on it at max or close to with no lag. I left out the case and speakers because they were cheaply made, my mother paid for it and she insisted on super cheap so I had to go pretty low-end for a few things.
I would also suggest Seagate instead of Western Digital for the HD, it's my experience that WDs come D.O.A. The one I bought for that particular PC did as well, but your experience may differ. Just a warning.
I also have a build for a processor-heavy PC that I built my grandmother. She insisted on a tiny case and a high-end processor but didn't care what else it had. Ended up about $500 and she can run just about every program she has on there at the same time with no lag or heating issues. It's just a netbox though, good for movies and multi-tasking but not for gaming/graphic programs.
Honestly, it's just about research. Make sure all the parts work together and meet your needs. Google is your friend here. Also keep in mind that shopping around will find you better prices and OEM/Bare parts will always be cheaper but offer no return policy or cords/driver discs/instructions usually.
Newegg has a very nice return policy and generally friendly/helpful customer service representatives, so they're a favorite. TigerDirect is also an option but they've been getting more expensive with less selection over the years. =/
...I've been building PCs for myself/friends/relatives for a while now and though I doubt I'm the best to give advice on it, the subject interests me a bit too much not to respond to this sort of topic. So sorry if I babble about it and certainly don't take my word without looking into things yourself. >.<