I kind-of look at it as if there are three categories of "affordable" mic pre's - (1) unuseable, (2) good enough for now, and (3) just plain good.
The mic pre in a Soundblaster card would be in the first category. Mackie pre's, Behringer pre's, Audiobuddy, Vtb-1, DMP-3, and probably the Aardvark pre's would fit into the second category. The RNP, Grace and Sytek would fit into the third category.
If all you have are pre's in the first category, it will always be worth your while to upgrade to a higher category. If you already have pre's in the second category, though, it usually isn't going to be worthwhile to upgrade to pre's in the same category. Sure, some of those are better than others, but laying out the cash for a marginal increase in quality usually won't be worth it. I'd say that your best bet is to work with what you have until you feel inclined to get something in the third category.
But that's just me. I'm sure others will disagree.