Edit: More info for clarification:
Mining client: the program that runs on your computer to mine
Mining pool: a group of people that work together, with the right kind of client that allows collaborative mining, to mine blocks
BitMinter is a pool that has its own browser-based client to mine. Your last post was about pools, whereas earlier in the thread, the discussion was about mining clients.
oh, my bad! I thought we were talking about the same
thanks for clarifying it
Edit: I was about to join 50BTC.com (a pool) but it says that I need poclbm-GUI miner (a client), right? So, do I need both?
If you go with 50BTC.com, or pretty much any other pool there is, then you will also need a mining client. BitMinter has the advantage that you can sign up and just use their mining client which makes things easy. That's not to say that it is necessarily better - it is just easier to get set up when you are new to Bitcoin mining.
If you decide to go with 50BTC.com you can use any mining client you like. Personally I'm a fan of DiabloMiner. I've tried several of them out there (DiabloMiner, CGMiner, BFGMiner, BitMinter) and DiabloMiner has consistently given me the best performance with my 6850s and 6990s. Note that I recommend you do the same and decide for yourself which miner you like best.
Something you will also want to keep in mind is that even the best pools can have downtime and you want to be prepared for that. Most of the latest miners allow you to setup backup pools that your miner will automatically switch over to if it loses communication with your main pool. For this reason you will want to have accounts with at least 2 different pools. This way if your main pool goes down for whatever reason your miner will switch to your backup pool and continue earning you Bitcoins. Much better than losing out on that time waiting however long for your main pool to come back online!