This is an article that talks a lot about what Xbox Live and PSN are doing to prepare their online services for the scores of people that will be playing GTA IV.
The article's quite interesting, because it's one of those few times you can really see where your money is going if you're a paying XBL customer.
It's also a great read because it explains why PS Online is free; because they don't do anything. In fact, it states that publishers have the responsibility of managing and providing the online play, which probably explains the quality.
This isn't to instigate any sort of "PS3 vs 360" arguments. So save it.
I honestly found it to be a good eye-opener as an XBL customer to see the level of effort they put into providing the best service they can (like doubling their server capacity over the past two months).