The best niches to be promoted on myspace are those related to entertainment, and I think gambling belongs to this category.
Additionaly gambling involves the hope of making money, and everyone want to make money right?
So I'd say gambling can be a good niche to promote on myspace.
Then for your other question, well whenever you hear people saying something is not worth it anymore, this very thing can be still working and making big bucks for some guys. I can only advise you to gove it a go and see what happens.
But beware : myspace is now considering mass promotion through bulletins and comments as spam. So you might need to behave differently.
Create a real person's (you) profile page and promote your product only on a part of it and in your blog, then use friend adding, bulletins, groups, forums and comments to promote your page.
Say something interesting and relevant, act like a real person so people will come and check out your page, see that you are a cool real guy, not just a marketer, and eventually they will see your product somewhere on your page or in your blog.
This way of marketing on myspace requires more time and work, but it is way more efficient in the long run : 1st you won't be banned, 2nd you will develop real contacts with some people and 3rd they will be more likely to buy your product if they feel like they know you as a real person, and therefore trust you.