At 1080p you are bound to face lags at sometimes. It is better you go for HD 7870 or a HD 7850. This will make sure that you can play at max settings for most games for about an year. And at high settings for the next year.
Unlucky, the bad thing about ps3's though, is not all games support 1080p. That's why I'm turning to my PC, and another advantage of PC is that the games generally cost less too due to licensing or something, and you also get better quality in PC too.
Are you talking about ram or video ram?
If you are talking about ram better get 2x4gB cards. You'll need it for gaming. You can only use more than 8 gB or ram with windows 64bit operating system.
If you are talking about video ram 1gB is good but better getb2gB if the price of the 2gB card is not too high.