The 5770 is about 10% faster and $20 more expensive on average. There are few reasons anymore to get a 5750 unless there is a nice deal or you are really on the edge of your PSU. I've seen the 5750 for $120 at times which is a nice price. The 5770 on average is $160 for the 1GB model which I would shoot for. If you don't plan to OC the card, just get a nice stock one, if you are planning to OC make sure you get one with the voltage unlocked and with a nice cooler. The MSI HAWK 5770 is a good choice or HIS IceQ 5 5770
RAM is RAM, it comes in different speeds and that is about all. Gaming RAM is just a marketing gimmick unless there is something technically different about it.