This is how I usually break things down in terms of trades, maybe it'll help. Forgive me in advance for writing so much, trust me that this should help though:
Mcnabb for Brees: Before the season started, obviously each of their draft rankings weren't near each other. Brees was the top fantasy QB or at least in the top 3 with Manning and Brady. This would have been a no brainer. However, of course, things change during the season. Bree's schedule isn't the most pleasant. Upon viewing it, he only faces 2 teams ranked in the bottom 10 passing defenses. I always look at how many teams in the bottom 10 passing defense, and rushing defense (depending on the player, wr, qb, or rb of course) to kind of gauge where they should post some sick numbers. Anything can happen of course, and things should change a little during the season, but I'd like to say when Brees faces Dallas's passing D and St. Louis's, he should post numbers close to, or in the 20's or better. Otherwise, he has some potentially rough games. Now, I'm sure he will do fine against many teams this year, since he is just simply very talented. In terms of him most likely having some huge monster games though, 2 games stick out at me. He faces 28th ranked Dallas's D in week 15 too, in the playoffs, so a nice game from him in that particular week comes right on time. That's obviously helpful. Mcnabb, however, is a little rougher in terms of schedule. He faces two teams as well in the bottom 10 passing D's. Dallas week 9, and San Fan week 15 (playoff time again, woo hoo! Lol) . San Fran's passing D is a little better than St. Louis's though. So when Brees faces them, Brees should slightly have a better game against STL than Mcnabb against San Fran. Already, in terms of pre-season rankings, Brees wins, and the strength of schedule will have to go to Brees since he faces a poorer passing D in St. Louis as his 2nd poor passing D. Now, in terms of teams, and which is more likely to pass, I'd have to go with New Orleans. Their defense is good, and will keep them in games. Their D is not as good as Philly's though, so there is slightly more of a possibility that they will be placed in a position for Brees to have to pass more to score. NO's rushing attack could be pretty sick, with Thomas, Bell, and Bush all involved. However, if you're down in a game, or need a quick score, you're going to throw, and Brees gets his #2 WR back this week in Lance Moore, so you add another weapon to his game. Mcnabb won't be in that much of a position as Brees will since his teams D is better. He'll probably run the ball more for clock management. Besides Jackson, who got no action last week by the way, and Maclin, who had one big week last week, there's no other sure threat, unless you count Kevin Curtis...riiight. One more knock on Mcnabb is him being injury prone as well. He's still not 100% , and it's a risk taking him because of that fact. Brees has a nice O-line, and shouldn't have those same worries. Simply put after this huge paragraph, Brees wins over Mcnabb. Your team could consist of Brees, Mcnabb and Palmer. I would try trading Palmer personally and keep Brees and Flacco, swiching them due to match-up. That's a nice 1-2 combo. Mcnabb and Flacco is nice too, but Brees changes it completely.
Moreno for Gore: Gore is the man in San Fran. Moreno is not in Denver. Buckhalter comes back this week, and will be 100% very soon. Gore has a higher potential to put up numbers and is a #1 back no matter how you slice it. Moreno is a #1 NOW. He won't be for the whole season. Plus, he's a rookie, and he's not conditioned to carry a 17 week workload. Gore is, and has done it in the past already. He's back from his injury, so feel free to deploy him for he rest of the season with no worries. Gore wins this battle.
Since your team has Flacco, Mcnabb, and Palmer already, do you really necessarily need Brees? If Mcnabb goes down again, you can switch between Palmer and Flacco, with Flacco being your #1. If he's fine for the year, you can switch between Flacco and him, with Palmer being your insurance if you're worried about Mcnabb, which isn't a bad idea. Brees is a nice upgrade, but do you need him? And is it worth giving up Gore in the process? You do have Ronnie Brown, who is fantastic, but who would you rather have as your #1 and #2 backs? Brown and Gore? or Brown and DeAngelo? Since Williams's production has been very iffy, and he's part of a poor team that will be passing a lot to catch up in the second half of games, I would go with the first combo of Brown and Gore. William's isn't a bad 3rd RB to have lol. He was picked in the first round everywhere. However, look at William's team. Yuck. Teams that suck, don't run all game. Look at St. Louis with Jackson, or the Titans with Johnson. It's hard to run the ball when you're down..... Brees is great, and would improve your QB's. You don't really have a need to though, and IMO stick with what you have. I've broken everything down, and if this doesn;t help you, and you do it anyway, enjoy your RB situation. Gore, Moreno, Williams, and Morris, won't take you to the promised land. With a team of backs like Brown, Gore, and Williams are your #3? Are you kidding? Do I really need to continue?