Shooting JPG is no problem with night photography - I tend to shoot JPG 90% of the time, even for stuff I sell or publish. RAW gives more latitude for correcting/altering in post processing, but get it right when you shoot it, and both formats can look great. Night photography is something I do a ton of...it's been a favorite and a specialty of mine for years.
Getting the white balance correct will be a big key for night shots. First thing you should do is determine what type of light cast you are experiencing - if your camera has live view, this can be a good tool to use in correcting white balance - or take a shot to review after changing the white balance mode to determine if you got it right. Often, cameras will even allow bracketing in white balance. Many times shooting city scenes or architecture in cities or public areas, sodium vapor lights are the culprit - they usually create a massively reddish cast. I find 'flourescent' white balance, or 'tungsten' white balance, modes to both compensate for this very well. Here's a quick example - the first shot was taken years ago with auto white balance, and I hadn't even realized at the time how much of a yellow/orange cast the shot had:
Only years later, as I became a little better at watching my white balance did I look back at the photo and realize it was way off on white balance. This past year I had the chance to reshoot at the same location - the angle and landscaping have changed a bit, but you can see the color of the building, and the sky, are both more to what was visible to the naked eye:
The building does have a dark green roof, the sky was blue, the trees on the cliff were actually green rather than yellow, the rocks are a more natural color, and so on.
Another thing worth mentioning - you said you had shot with wide open aperture. Many times, lenses will get significantly sharper when you stop them down a bit - so you might find your results get better if you set the aperture to F5.6 to F10 or so...let the shutter go longer to compensate if the scene is very dark. I'd prefer to shoot a 30 second shutter at F8 than to shoot a 5 second shutter at F2.8 on an F2.8 lens. You might find you can get pretty solid results by shooting night photos using Aperture Priority mode on your camera. Set the aperture in the lens' sweet spot - think around F8 as a beginning point, and adjust slightly up or down if you need to - set ISO to lowest level, and let the camera choose the shutter speed. Don't forget to set the white balance first. And when shooting on a tripod, use either a remote shutter release, or set the self-timer to snap the photo so you can be hands-off. Turn off any stabilization systems in lens or body. If you have a mirror lock-up function, use it - sometimes it's built into the 2-second timer.
You'll get sharper, crisper results, better depth of field, and with the right white balance, you'll get good colors at night. Sodium lights will still show up reddish or yellowish in the shot, but they won't create that color cast that makes the whole thing red - blue skies will still be blue, and green trees will still be green:
Hope that helps!
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!