If you only have Nascar Thunder 2004:

Reach EA Bio level 16 to unlock the All-Star race at Lowe's
Reach EA Bio level 18 to unlock the Bud Shootout at Daytona

If you have Nascar Thunder 2004 AND Madden NFL 2004 saved games:

Reach EA Bio level 10 to unlock the All-Star race
Reach EA Bio level 12 to unlock the Bud Shootout