3dMark2000 and 3dMark2001 are strongly dependent upon hardware T&L
UT is not
Do not use an artificial benchmark like 3dM2k1 to compare systems with different components. Only compare it to systems with like components so you know if you have something "amiss", i.e. slower driver revisions, issues, etc.
My recommendation:
1) Play the games you like
2) Up the resolution and detail level as high as you can take it
3) Make adjustments so that the smoothness and stability of the game is as necessary
4) Decide if the 5500 is for you
If it can hang in your favorite games with visual quality that makes you say "aw, cool, this is good!" then keep the card.