First off...it's been mentioned here, but worth repeating: the key to good "night shots" is a tripod - virtually any camera from the lowliest P&S to the highest full-frame DSLR can do those quite respectably. Any half-decent P&S that allows shutter speeds of 15-30 seconds can pull off large print stunners, no noise, super sharp details, etc. That's because good night shots are taken on a tripod with the lowest ISO setting, middle apertures, and a long shutter speed...something even P&S cameras can do.
The key to "night action shots" is markedly different...for those, you need fast shutter speeds, and at night, that means wide open apertures and/or very high ISO settings. P&S cameras can't do these very well, no matter how good the photographer, and for these, you would need to move into a DSLR to get good results. And even with a DSLR, some will be better than others, and the lens will still be a big factor.
I'm willing to bet the OP's 5-year-old Sony could probably give some surprising results. Almost all Sony models but for the smallest pocket models have the ability to shoot for multiple seconds...which means most models can pull off very nice night shots. I've been a big fan of night photography since 2003, and I've worked with 7 different models of Sony cameras, from high-end P&S to pocket compacts to DSLRs. They're as good as any other cam on the market for night shooting. For an idea, check out my night photography gallery here:
http://www.pbase.com/zackiedawg/night_photography
There are 6 pages of thumbnails in there from all different cameras...however the first 2 pages are all taken with Sony P&S cameras. All it took was a tripod (or putting the camera down on a ledge, fence, wall, table, trash can, or other level surface) and the 2-second timer.
Decide what you really need, how much you want to spend, and how much compromise you are willing to make in portability and convenience. While DSLRs are the ultimate, and I wouldn't go anywhere without mine, they too involve compromise - after all, you can't fit any DSLR in a chest pocket and usually they come with quite the collection of lenses you've acquired. It just may be that a P&S camera will fill 90% of your needs, and the 10% it can't cover can be outweighed by the convenience and portability, as well as price, enough to make a good P&S the best choice for you.
When it comes down to it, the photographer still makes the biggest difference, not the camera. Fine art and gorgeous photos can still be taken with a pocket cam wielded by a great photographer, and a great camera can still create junk in the hands of someone not skilled. I'd recommend trying out some slow shutter techniques with that 5 year old Sony, and you may be surprised how good it can actually do for night shots. That may help guide you to the decision of what to get next.