You can easily and cheaply answer this for yourself, right now.
Take a file you think you might want to make a poster out of. Do whatever post-processing you might do, including scaling it up to the full size of the poster.
Now, crop out a portion equivalent to the largest size you can cheaply print today, print the crop, and evaluate said print.
The answer, of course, is, "it depends." It depends on the quality of the capture, your workflow (including your skills at post-processing and enlargement), and also your standards.
However, chances are excellent that your posters will be about as good as everything else you do. If you're happy with your current work, you should be happy with your posters.
One last point to consider: a 5DIII file (the only type I have on hand) will make 40" x 60" prints at about 100 ppi, which is a typical monitor resolution. If you're happy with what you see when pixel-peeping, the print will look exactly the same. Now, zoom to 125% and you've got the equivalent of a 75 ppi file, which will print at 50" x 75", which is basically life-size for a head-to-toe portrait.
Any flaws in your technique will show themselves to anybody standing at a conversational distance from the poster, but, if your technique is good, the end result should be stunning.
...this assumes no (significant) cropping, of course....
If you walk through some similar math (Photoshop's Image Size and Canvas Size dialog boxes make it super-easy), you'll find that, in the big scheme of things, there's not a whole lot of real-world difference between 18 megapickles and even 36 megapickles. With poster-sized prints you're finally getting into the realm where the differences are visible in side-by-side comparisons...but, honestly, lenses, shooting technique, and post-processing are going to have a much bigger impact on final print quality than mere megapickles. A bad tripod / excessive wind / too slow a shutter speed can very quickly turn a 36 megapickle picture into an effective 18 megapickles, or much less.
That would bring me to my final point. If the gear you have won't cut the mustard, don't get something else in the 135 format. Assuming it's not your technique / lenses / whatever at fault, what you really need is a bigger sensor. Medium format should do the trick...damned few actually need medium format, let alone large format, but poster-sized prints is one of the cases where medium format starts to make more sense than 135 format.
And, no -- that has nothing to do with megapickles. You know how there're these 20-megapickle point-and-shoot cameras with a sensor the size of your fingernail? Would you even give a moment's thought to substituting one for your 1DX?
Medium format is to 135 format as 135 format is to APS-C, and automatically brings with it a comparable set of improvements (and hassles), regardless of sensor resolution.