As I understand the two, the 5670 should be, in general, slightly faster than the 9800 GT, as a result of its higher memory bandwidth. The 9800 GT, on the other hand, should be superior in the areas of AF, and AA. This means that you will receive higher framerates with the 5670, but when framerate isn't an issue, games may look slightly better with the 9800 GT, as you can have AA and whatnot turned up higher.

It's something of a crapshoot as to which you prefer. It's also hard to say "card x is BETTER than card y" when their specs are anywhere near similar, and, in the case of these two cards, they're very similar. If I had to pick one I'd go with the 5670, because it supports DX11 and uses roughly half the power of the 9800 GT. Still, regardless of which one is "better," the point of my response to you was just to point out that he shouldn't return his 5670 to get a 9800, as he's likely to get a different, but not better, experience.

I definitely agree with your assessment that it's better to spend just a little more and get a card you know you'll be happy with, though.