Hmmmmmm...OP is upgrading from a 25W amp and thinking 100W+, maybe a single 15" and under $1,000. Some of the suggestions in replies are possibly overspec.
(However the advice to err on the side of too much power is wise).
Three options I would suggest.
For jazz gigs I use a 100W Roland Cube. Cost me $600. But will run out of headroom for rock gigs and your profile says that is your thing.
For rock / blues I use a Peavey 200W single 15" combo with a Hartke 2x10" extension speaker, and a SansAmp in front to get some decent tone (the preamp on the Peavey is pretty ordinary). If you go this route, be aware the Peavey is heavy, and you will need to add handles (cheap at Jaycar provided you are ok to install them yourself). Also be aware that Peavey is not prestige gear, but it is reliable and cheap. You will be able to do a 2nd-hand Peavey + new Sansamp for maybe $700-750. The 2 X 10 box can be added later if you can't afford it. You can even get just the Peavey and add the Sansamp later if money is tight, but the sound won't have GRRRRRR.
Finally you can look at the entry-level Ashdown component gear. You can buy their 300W head + a 1x15 or 4x10 new in your price range. Entry level combos by several manufacturers also exist in that price range. It depends what you want.
And of these options is a step up from what you have, and are fine for entry-level gigging. Of course if you have more money you can get something bigger and better.
And if you get a short list of amps, make sure you look on Harmony Central for reviews (though read them carefully since anyone can post a review there, so you have to filter out the reviews where someone is raving about their new toy while being blind to its faults, or is reviewing because they had a bad experience and they want to pay out).
