300 DPI? 300 Dot Per inch? :P
You're talking about the resolution no? Well I've done other posters using the same size / resolution and printed it before and it turned out fresh :P.
If there's anything I should focus on do let me know because well this what i use:

Best Quality is 300 DPI+ (so yes resolution), 16/32 bit colors, and then use CMYK color instead of RGB