Sorry, I happen to strongly disagree with JBI's advice:
The i5-760 is better than the x3-455 in almost every aspect INCLUDING single threaded applications. You should have a look through this article
that discusses bulldozer and look for your competitors on each chart. Here
is a direct comparison between the 760 and the 455 that backs up my point. Also, do not just assume that you'll be able to unlock the 4th core on a x3-455, I did a budget build for my brother with one in June 2011 and couldn't get the 4th core unlocked on an asus board.
AsRock is a very good company and they make boards that consistently get very good reviews (preferred over gigabyte often).
A 6850 will be generally better than a regular 460 1GB, let alone an SE model. The 6850 is also cheaper than a GTX 460.
I do agree that a 5400 rpm hdd will not be "optimal." In light of the high cost of mechanical hdd's I would support perhaps grabbing even a 90GB SSD. You can get these for around $110 on sales and would be a HUGE improvement over a hdd. You can then use the hdd as a storage drive.
I would recommend checking out TR's System Guide
or perusing the TR System Builder forums for budget build threads to get ideas on a from scratch build. That Chieftec PSU makes me a little nervous, I would go with something from Antec or Corsair. Also, if you're building yourself a system, go with 8gb of ram. You may not need it right now, but its only about $13 more than a 4gb kit.
Lastly, if you're spending....$500? for your friend's used rig you may be paying too much, I would try talking him/her down. TR's Econobox build for ~$600 would easily outgame your friends computer. (Obviously the econobox price doesnt include an OS, etc.) Not sure what you're using the computer for, but hey, don't spend more than its worth just because its from a friend. Your friend's system hardware (that you've listed) only cost about $650 new. (I guessed the case price at $50)
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod