The 6750 is a faster and more energy efficient card than the 9800 GT. When the 9800 GT was new, it was a pretty decent card. By today's standards, though, it's slow. It is listed as the minimum requirement card for BF3. Because of this, an HD 6750 is a slight upgrade.
I do feel your pain a little. When I RMA'd my 8800 GTX, I got back a 9800 GTX+. I sent them a card I paid $550 for, and got a card back that at the time was $220 brand new. However, you have to remember that the 9800 GTX+ was faster in most scenarios. Because of this, it was an "upgrade" regardless of perceived value.
Anyone that says you should have gotten a 7950 or a 7870, or some other modern work-horse card clearly does not understand what XFX's warranty is. It's a lifetime warranty that will replace a defective card with one of comparable power. however, that means that after 4 years, what was once powerful is no-longer so, so you will get a card that's further down the line. It is still an upgrade though, which means XFX complied with the warranty terms, and the OP got an upgrade. It may not have been what he wanted, but that's usually how warranties work.
As someone else pointed out, if XFX were to replace every old defective card with a newer card of comparable value when the other card was brand new, they would never make any money. Everyone knows that an HD 7850 runs circles around a 9800 GT. If both cards cost the same to manufacture, then that means XFX makes no money after the first RMA, and actually loses money after the second. NO COMPANY in the world would do that, as that would mean they would never make a profit. However, if you replace a card that cost $100 to manufacture with a card that costs $30 to manufacture today but the cheaper card is still faster, the company is STILL making money, and the consumer is happy because they get a faster card.
That's why XFX is able to offer a lifetime warranty without losing money. Edited by Mad Pistol - 1/14/13 at 9:29pm