Below I'll paste an informal analysis of the Enough lobbying letter (recommended Act). It's pretty informative to see how people familiar with the many facets of the issue think through any potential action:
The letter is good, in that it spells out the actions Bush and the Security Council could take – but none of them are that simple, which is why nothing has yet been achieved. Firstly, bringing forward a resolution for the Security Council – this has been needed since 2003 – has not yet been possible because of China. (who is on the SC, and has massive oil interests and investments in Sudan as they get a large proportion of their oil for their relentlessly growing industries). China thus far has said that they would veto. France, also on the SC, has troops and oil interests in neighbouring Chad, into which the conflict has now spilled over. France is scared that through the conflict, China will get a foothold into Chad and start getting oil from there also. The UK ambassador to the UN (who I know personally) has been trying for 3 years now to make some progress on this issue. But Darfur and Eastern Chad are just desert, there are very few roads and infrastructure and therefore it’s very hard to gain humanitarian access to the region, let alone impose a ‘no fly zone’ or to monitor who’s doing what to whom.

Another important international debate is the role of the International Criminal Court (look out for my June CR Sector news commentary on this issue!) The USA hasn’t signed up to the ICC, due to the fact that so many of its leaders and military commanders would be accused of war crimes. They don’t agree that an international body should have the power to judge how the US conducts matters, even if they are abroad. SO that means that they have no power or right to enforce the indictments that the ICC announced earlier this year for the arrest of 2 of the military commanders (who were also in the Sudanese government) who are still at large.

The other thing the letter suggests is sanctions. Again, not that easy as these could end up hurting the already empoverished people of Sudan. I think they would obviously target them on the GoS and companies, but still there’s always the risk that the people working for those companies on less than $1 a day could lose their income….

On balance, though, I think you should send the letter, as it never hurts to keep up the pressure – because as you can see this issue needs to be dealt with at the international level, and the Government of Sudan who among many other factors that caused the conflict (that can be a separate email if you like!) are responsible, need to be stopped. And if you can hand write the letter, I’ve found that it has more impact.