A 3DMark2001 score of about 6000 is actually quite low by today's standards. For a 'good' responsive gaming system you need a card that will score in excess of 12,000, and preferably one which will score 15,000 or better. Even a previous generation Nvidia GeForce4 Ti4200 card will score at about 12,000.
The 'cut-down' versions of Radeon cards perform at a lower level than that, as they have reduced memory addressing bandwidth and cant perform calculations with enough efficiency as a result. The 'cut down' versions of Nvida FX cards, on the other hand, are usually simply clocked lower, and are closer to their 'big brothers'.
Bur 3DMark2001 id a DirectX8 tool, and most new games are DirectX9. NVidia FX cards are NOT as good as the Radeon 9xxx equivalents for DirectX9 features, because of the way it is implemented.
Also, the 'letter designations for Radeon and NVidia cards are used differently. For Radeons, an 'XT' is an overclocked card. For the NVidia FX an 'XT' is an UNDERclocked card! The FX5900XT is the best of the best with 'budget' lines, because of the components used. It's basically a card that was designed specifically to move excess stocks of 'good' components in preparation for the introduction of the new NVidia 6800 generation of display cards.