Might as well save up and buy one great pair - better then trying to upgrade.
I'd recommend Eternal 811 or a pair of Flat Heads to start you off - great denim, great construction, elegant simplicity. Or for a bit more flare, try a pair of Sugar Cane's or Samurai Jeans.
Here are my latest pick-ups, also posted on mynudie.
First pic (clockwise):
Aika 'Jin' jean
LVC 1967 505
Edwin Vintage Collection 506
Enishi 'Yoshi' jean
Second pic (left to right):
Studio D'Artisan 'Clipper' jean
Oni Blue fit 18oz streaky jean