It's too early to know anything for sure, but the Giants are looking like at least a playoff contender after their impressive sweep of the Astros. To be fair, the Astros aren't a very good team, but they're not total pushovers either (I think). While they can feel confident in Tim Linecum pitching well his next time out, Barry Zito (who pitched well in his first start) is a different matter after his rocky tenure with the Giants.
The last time they won their first 3 games was 2003 (they went on to win their first 7), when they won 100 games and the NL West. They also won their first 6 in 2002, when they came within a six outs of winning the World Series, and their first 3 in 2001 when they finished only 2 games behind eventual World Champion Arizona Diamondbacks.
They'll be a lot like they were last year, when they had the 2nd best team ERA and scored the 5th fewest runs (and had the 2nd fewest home runs). They have some good tests coming up, facing the Braves at home, then the Pirates at home and the Dodgers on the road.
Update: Eduardo Perez thinks the Giants will score enough runs to compete and that Aubrey Huff will be a key asset:
The Giants' starting five are going to be fine. San Francisco's question mark is not going to be pitching; it's going to come from the cleanup spot. Can Huff handle the job? He definitely can.