Buyers or sellers?
The Giants are contenders for the first time in a few years and are looking to acquire additional pieces in their push for the playoffs. They still have a small chance to win the division and may be the favorites to win the wild card race so they will be buyers. Their pitching has been phenomenal but they are still lacking offensively, ranking 13th in the National League in runs scored.
Short-term need
Bats, bats, bats, and more bats. The Giants need as much offensive firepower as they can get. Other than Pablo Sandoval the Giants don't have any great hitters so they could use upgrades almost anywhere. Realistically, the most likely spots that could be improved in a trade are the middle infield, first base, and one corner outfield spot.
The Giants have been involved in rumors throughout the year for middle infielders like Freddy Sanchez, Jack Wilson, and Dan Uggla. The Giants don't currently have a guy who needs to start at either position so they could give at bats to any of those guys. It may be in the Giant's best interests to try to wait the Pirates out and get either Sanchez or WIlson for cheap because the Pirates need to shed their salaries.
As far as power bats, the Giants have been linked to various names including Jermaine Dye, Adam Dunn, Nick Johnson, and Matt Holliday. The cost for these guys could be too much for the Giants, who are trying to keep their best young players.
One thing to consider is that general manager Brian Sabean is in the last year of his contract and is on the hot seat. If the Giants don't make the playoffs this year his job could be in jeopardy, so it may be in his best interest to sacrifice a bit long term in order to win this year. If the Giants at least make the playoffs his job will probably be safe.
Long-term need
The Giants have gobs of pitching both now and for the forseeable future. The future of the team depends on how the young bats develop. Even in the best case scenaio, the Giants don't have a lot of talented middle infielders and that should be a spot the organization is focused on improving. Guys Emmanuel Burriss and Kevin Fransden are stop gap options and there really aren't any clear starters in the minors for the next few years. The outfield has quite a few decent players but there's not a clear stud out there either.
Once Buster Posey is up the catching situation should be resolved for quite some time and Angel Villalona and Pablo Sandoval could evenutally be corner infield fixtures together. But the focus should be on getting middle infielders and high upside outfielders.
Big leaguers on the market?
Jonathan Sanchez has been rumored as a piece in numerous deals. He could be involved in a trade for Freddy Sanchez, Nick Johnson, or a number of other players. However, the recent injury problems of Randy Johnson and Sanchez's no-hitter make it more likely that he stays in a Giants uniform.
Other than him, most Giants are either too young to be traded as the organization doesn't want to ruin the young core, or they are veterans who will be needed to help the Giants reach the playoffs.
Minor league strength
The Giants don't have the deepest minor league system but they do have a really good group of top prospects. Pitchers Madison Bumgarner and Tim Alderson have both been dominant in the minor leagues and have the potential to be at the front of a big league rotation. The Giants also have plenty of pitching depth and "Have a knack for finding spare pitching between the couch cushions" as prospect guru John Sickels put it.
They also have an outstanding prospect in catcher Buster Posey, who should be their catcher for years to come. After that there is not another great prospect but there are still plenty of potential impact players in guys Angel Villalona and Connor Gillaspie. The Giants main areas of weaknesses are up-the-middle talent (minus Posey) and hitting depth.
Take on short-term money to win?
Henry Schulman of the San Francisco Chronicle recently wrote, "The money might be there to make a deal, but the cost in prospects to acquire an impact bat might be prohibitive". The Giants should be able to take on a contract as long as it's not too expensive and they don't feel the prospects they are forced to give up will hurt their future.
For more coverage of the Giants, check out McCovey Chronicles.