As Bart said, posters have a certain viewing distance. Even more so when they are printed at low resolution in bulk for marketing purposes. As long as your image is well shot and post produced, there won't be a single problem with large prints.
Talk to your clients and have them explain what kind of end use your images will have, then work your way towards that goal.
As for your equipment: sharper lenses, better light (quality flashes, softboxes, reflectors), post-processing and good technique will help you a lot more than just sinking money into a higher resolution body. You can gradually buy into a more modern system by getting the job done with what you have. The lenses and other equipment will stay with you if you save up for a Canon 5Dsr, so it's a good investment in the long run.
As an example, check out these two shots
at 100%. One is from a phone (8MP, cropped), the other is from a D200 (a 2005 "vintage" camera with 10MP, cropped). The phone shot has no sharpness applied to it, and the other only minimal (for web), so with enlargement and better sharpening I could get large prints that are perfectly acceptable for advertisement.