5400rpm is pretty slow man. Get a GTX 660 ti or a GTX 670. Pick up ram of choice (2x4 at 1600mhz should be enough) and then you case of choice. I would also recommend to get a new hardrive lol. Throw any CD drive you can find in there, not really important.
I realize that the hard drive is slow, it just wasn't the most important thing to replace right now. I was wondering how best to spend the $400-$500 on the parts I would need to complete this build. If I go with a GTX 670, that's a majority of the cost. Is that the best approach in this situation?