Whatever you want to invest in a RB take about 70% and invest it in the o-line (unless you are already thinking late round pick)...with a quality o-line you can get very serviceable RBs, and then once the o-line is solved, go invest a bit more into the RB. That's how I'd approach it.
Last year they tried to get Cyrus Grey in the 6th I think....just find some guy like that that can pass block, is a determined runner, smart, and reasonable athleticism. Its that last aspect that vaults players up the board, but you can do just fine with a 4.5 RB as opposed to a 4.3. Would like 4.3, but would take a big nasty linemen 7 days a week instead.
A good line makes average skill players outstanding production players, an average line can be made to look good with HOF elite skill players, but a poor o-line sucks for everyone all around - RBs, WRs, TEs, and of course pinata QBs.
I'd like to think Murray is a fluke thing given his track record at Oklahoma. Felix unfortunatley is not.