The price that the jeweler gave you for the diamond is irrelevent; get a GIA or AGS certificate for it (any reputable jeweler selling a diamond for that price will have it certified), and find out a little more about the stone. Then compare it to stones you can buy at places like
DirtCheapDiamonds or
SuperbCert. There are other reputable diamond dealers online; I recommend looking at the
DiamondTalk Forums and becoming a little more knowledgable about diamond quality before you plunk down a huge amount of money on one.
What do you mean she "registered" at Helzberg? Did she pick out the exact ring already? If so, you're screwed; you might as well buy it right now. On the other hand, if she just told you what kind of ring she would like (from a design standpoint), you can definitely get a better deal from a private jeweler (not a big chain like Helzberg), or online.
And FYI, the cut of the diamond matters infinitely more than the exact color or clarity (many people can't tell the difference between a colorless and a near-colorless, or notice imperfections that can only be seen with a jeweler's loupe). The cut is what defines a diamonds light refraction, or luster. An ideal or super ideal cut diamond will not only catch her eye a lot more, they will also appear bigger in size.
I bought a single .5 carat ideal cut center stone (VVS2, I), and two matching .25 carat ideal cut side stones (VVS2, I), plus a white gold setting, for under $2000. I may have had a price advantage because my fiance wanted a three-stone ring, and 1 carat in three stones is cheaper than a 1 carat solitaire. But because the stones were all idea cut (AGS 000), they appear noticeably bigger than their carat size gives away.