The Accursed Bow of the Elite is an iLevel 200 bow that drops from Instructor Razuvious in Normal Naxxramas. This is a 2.80 speed bow that provides 129.6 DPS, 28 Agility, 19 Stamina, 16 Hit Rating, 21 Haste Rating and 40 Attack Power.
The Nerubian Conquerer is an iLevel 213 gun that drops from Kel'Thuzad in Normal Naxxramas. This 2.90 speed grun provides 141.6 DPS, 31 Agility, 30 Stamina, 21 Crit Rating and 64 Attack Power.
Final Voyage, which drops from the Four Horsemen in Heroic Naxxramas, is an iLevel 213 crossbow. 2.80 speed, 141.6 DPS, 18 Agility, 33 Stamina, 16 Crit Rating, 25 Haste Rating and 62 Attack Power.
Arrowsong is an iLevel 213 bow that drops from Patchwerk in Heroic Naxxramas. With a 2.80 speed, it provides 141.4 DPS, 21 Agility, 37 Stamina, 16 Hit Rating, 20 Crit Rating and 62 Attack Power.
And finally, the Envoy of Mortality is an iLevel 226 gun which drops from Kel'Thuzad in Heroic Naxxramas. This 2.90 speed gun gives you 154.8 DPS, 36 Agility, 30 Stamina, 26 Crit Rating and 72 Attack Power.
The Rifle of the Platinum Guard is an iLevel 219 gun that drops from Ignis the Furnace Master in Normal Ulduar. With a 2.80 speed, this gun offers 147.5 DPS, 42 Stamina, 24 Hit Rating, 29 Crit Rating and 31 Armor Penetration Rating.
The Golemheart Longbow is an iLevel 219 bow that is a zone drop in Normal Ulduar. This 2.90 speed bow offers 147.4 DPS, 33 Agility, 39 Stamina, 17 Crit Rating, 22 Haste Rating and 46 Attack Power.
Avalanche is another iLevel 219 bow. This 2.90 speed bow can be found in the Normal-mode Cache of Winter, which becomes available after defeating Hodir, and provides 147.4 DPS, 26 Agility, 39 Stamina, 16 Hit Rating, 66 Attack Power and 17 Armor Penetration Rating.
Siren's Cry is an iLevel 232 bow that drops from Auriaya in Heroic Ulduar. This bow's got 2.80 speed, 161.6 DPS, 38 Agility, 36 Stamina, 25 Crit Rating, 22 Haste Rating and 54 Attack Power.
Last but certainly not least, Giant's Bane is an iLevel 232 gun that can be plundered from the Heroic-mode Cache of Living Stone after defeating Kologarn. This 2.90 speed gun offers 161.7 DPS, 38 Agility, 33 Stamina, 27 Haste Rating, 54 Attack Power and 21 Armor Penetration Rating.
Now that we know what we're dealing with, let's get into a few comparisons.
First, you'll notice that the best weapon from previous content, the Envoy of Mortality from Heroic Naxxramas, still provides more base DPS than the introductory weapons from Normal Ulduar. Additionally, Envoy provides more Agility, Crit and Attack Power than the Golemheart Longbow, Rifle of the Platinum Guard and Avalanche. In short, you can continue along a linear progression route from Heroic Naxxramas into Heroic Ulduar without missing a potential upgrade. However, it's worth noting that both the Rifle of the Platinum Guard and Avalanache provide Hit Rating and Armor Penetration Rating which Envoy lacks.
As an aside, I'd like to highlight the fact that the Rifle of the Platinum Guard lacks both Agility and Attack Power. As such, it's certainly not an ideal upgrade for any Hunter; however, it's listed here primarily because it does have higher base DPS than the previous Normal mode items and doesn't have any stats (such as Strength) that would identify it as something Hunters should specifically avoid. Additionally, it has the highest amount of Hit Rating, Crit Rating and Stamina of any ranged weapon found in Wrath raid content, which is at least noteworthy.
However, a comparison of the five best weapons from each level of content is of greater interest: the Nerubian Conquerer vs Avalanche and Envoy of Mortality vs Siren's Cry and Giant's Bane.
The Nerubian Conquerer, as stated above, provides 141.6 DPS. Thus, Avalanche constitutes a 5.8 base DPS increase right off the bat. Avalanche also provides an increase of 9 Stamina, 16 Hit Rating, 2 Attack Power and 17 Armor Penetration Rating. However, by making the switch from the Nerubian Conquerer to Avalanche, you would lose 5 Agility and 21 Crit Rating. So what does that all mean in terms of total DPS? Avalanche would constitute a marginal DPS upgrade overall: roughly 5 DPS when all is said and done.
Now for the "big guns", so to speak. For Dwarves and Trolls, it's worth taking your racial abiliies into account here since Siren's Cry and Giant's Bane are largely similar in stats.
Both Siren's Cry and Giant's Bane provide roughly 7 more base DPS than Envoy of Mortality right from the start. Additionally, both Siren's Cry and Giant's Bane provide more Agility, Stamina and Haste than Envoy. The major difference you'll notice when making the jump from Envoy to Siren's Cry or Giant's Bane is that both of the new weapons will drop your Attack Power by 18. Giant's Bane also constitutes a loss of 26 Crit Rating (Siren's Cry is only a loss of 1 Crit Rating). After converting the differing Attack Power and Agility values to raw DPS, you'll find that both Giant's Bane and Siren's Cry will provide roughly a 6 DPS increase over Envoy. The difference in Crit Rating between Giant's Bane and Envoy constitutes less than 1% Crit Chance, making it a relatively inconsequential change. Again, this makes either choice a marginal upgrade over what you've currently got.
Whether you do Normal or Heroic, Ulduar does provide some ranged items that will boost your DPS, though it does seem that the differences between the previous items and the new aren't as vast as we might like.