You've got a lot of latitude when choosing a font for headlines. Both serif and sans serif work and you can get away with a fonts that have more "personality" than readability.
Body text, however, should be optimized for readability so your choices are more constrained. I don't think there is a huge difference in readability for serif vs. sans serif fonts for body copy. Ask a couple of designers and you'll get a couple of different opinions. Some will say that serif fonts are better because the serifs "lead the eye along". I'm not sure I buy that, but I do prefer serif fonts for body copy. But if you use a serif font, make sure it is big enough. At small sizes the serifs don't render well and get in the way of readability.
I've heard that Arial works better as a display font instead of a font for extended reading, which I tend to believe since it looks so much like Helvetica. If I choose a sans serif font for body copy, I choose Verdana which was specifically designed for the screen. But really, I don't think Arial is necessarily a bad font for body copy. I don't have any objective reason to say why you shouldn't use Arial.
Also, the header font and the body font should contrast, so if you have a serif header font it's usually good to have a sans serif body font and vice versa.
Body text, however, should be optimized for readability so your choices are more constrained. I don't think there is a huge difference in readability for serif vs. sans serif fonts for body copy. Ask a couple of designers and you'll get a couple of different opinions. Some will say that serif fonts are better because the serifs "lead the eye along". I'm not sure I buy that, but I do prefer serif fonts for body copy. But if you use a serif font, make sure it is big enough. At small sizes the serifs don't render well and get in the way of readability.
I've heard that Arial works better as a display font instead of a font for extended reading, which I tend to believe since it looks so much like Helvetica. If I choose a sans serif font for body copy, I choose Verdana which was specifically designed for the screen. But really, I don't think Arial is necessarily a bad font for body copy. I don't have any objective reason to say why you shouldn't use Arial.
Also, the header font and the body font should contrast, so if you have a serif header font it's usually good to have a sans serif body font and vice versa.