I was getting 7 mhash/s on GUIminer.. but only 0.3 on bit minter?
As far as I can tell GUIMiner may have been failing, 

The reason why Bitminter says "Warning" is because CPUs are not effective for mining. (I have an Phenom II X6 @ 3.6 Ghz and get the same as you), My graphics card (ATI 7770) can only just mine them now. 

How many cores is bitminter set to use?

My recommendation could be for you to mine litecoins and then exchange them for bitcoins. Litecoins mine better on the CPU.