It depends upon who's going to buy your photographs. If they are small prints or put up on the web that's one thing, if they are going to be involved with print adds, advertising where there will be a commercial artist using them in photoshop and including them in a composite, you'll want the best camera you can get and the reason much of this is shot with medium format cameras.
To date, one friend/client I've shot for feels the 1DsMKIII is just adequate but the 1DMKIII was not once he had to do any cropping or pixel moving at all in Photoshop, this was for both catalog, posters and web production.
But for things like many portraits and weddings where the outcome is now going more to slide shows in addition to prints, then from what I've seen, cameras like the MKIII and 50D are great. Obviously sports dictates the use of cameras like the MKIII and D3.
But you've got to start somewhere and I get asked to shoot portraits every week by people who just like what they've seen of my work and haven't a clue what my camera will or will not do. I've declined because I don't want to do "work" right now, just shoot people for the love of it and not turn it into a business thing again, at least not yet, I'm retired and love not having a schedule and even more so no having clients LOL.
I've a friend who is just discovering what changing the ISO does, he's been making money from photography for several years and this boggles my mind sometimes but he leaves his Nikon on ISO 200 and gets the shots because he's great with people and composition and has a good eye in general so he's figured out a way to be successful, he has two D300's some really good glass and both strobes and speedlights. So in a way there are no rules if you can get what the client wants.
The world is changing, most of the people who approach me now are not regular types of those you would go looking for as clients. I think a person could do well just shooting really good portrait and product photos for peoples websites. I get those requests a lot, granted they are not used to paying professional prices but it might be a great place for someone to get a business started, even if at first it's just about making enough to get your gear paid for and getting some experience. When you then become in demand - expand, both your horizons and your prices.