I was getting 7 mhash/s on GUIminer.. but only 0.3 on bit minter? I'm really new to bitcoins. I have a good computer, cpu based at least. 8 core AMD processor with 4 ghz a core with decent amount of cache. I wanted to make some extra money and use my computer to get some bitcoins. Knowing nothing about the industry first thing I did was download wallet and GUIminer. Guiminer keeps taking all the cpu and generates up to 7 Mhash/s However it proves really ineffective, taking all that cpu and showing absolutely no progress on the BTC guild website. So I looked for an alternative method of bitcoin minig that will result in higher reward. I came to bitminter, downloaded their client. When I ran their client, it generated a 0.3 Mhps. This was really disappointing and I don't know why.
Well why is my mhps so low on bitminter, and where should I mine bitcoins? 

Also it says "warning: electricity-wasting slow cpu" but I have a beast CPU
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.