Hi Jerry,
As Summer mentioned, a lot of the decision will come down to what you and your sweetie's priorities are. While "I" color is not among the "whitest" of grades, it is perfectly suitable for a ring, and shouldn't be discounted out-of-hand. There are a lot of people who would start to hesitate after passing the "G" level, but in reality an I color stone isn't exactly an eyesore either.
A couple factors play into this decision. First, the brilliance of an ideal cut stone will tend to drown out a lot of the "off" color of the diamond. If the stone truly is of excellent make, it will sparkle and look beautiful even with the lower color grade.
Another factor that you will probably want to take into account is the type of setting the stone will be living in. The yellow of the diamond will be much less noticeable if set in a yellow gold ring since contrast between the (very yellow) gold and the (slightly yellow) diamond will likely make the diamond look whiter. Conversely, a platinum (or fine white gold) setting may look whiter than the diamond, which would be decidedly bad for the overall appearance of the ring.
Personally, I would probably look for something a bit whiter, but if size is important and the budget is already strained I wouldn't worry too much about buying a slightly less valuable color grade. Sometimes concessions must be made to size, and as long as the cut is excellent and the setting will flatter the stone there's no reason to kill the budget along the way.
My $.02...
-Tim