Before recommending a lens... You mentioned that you had some 'old'
lenses that you're going to keep (I assume they're all still EF
mount lenses?) Which ones are they? Also, what kind of shooting do
you normally do? Can't you just keep those lenses for use on both
your 300D and 3000? What were you using as a 'general purpose' lense
on your 3000?
I guess a couple of good 'general purpose' lenses could be the 50mm
f/1.8 or the 28-105mm.
The 50mm f/1.8 is very cheap but also very sharp and fast.
Definitely the sharpest non-L lens available.
The 28-105mm has a reasonable focal length coverage but still is not
too expensive. If you can find the older (now discontinued) 28-105mm
f/3.5-4.5 it is much, much better than the newer f/4-5.6. B&H in the
US still has them in stock. Otherwise, check the trading post and ebay.